Chapter 4

Examination of the Genital Tract and Pregnancy Diagnosis

 

   

 

Examination of the Genital Tract and Pregnancy Diagnosis

I. Resources Material

A. Reading Assignment

1. Arthur et al, Chapter 3, Pregnancy and Its Diagnosis pgs 60-104

2. Roberts, Chapter II, Examination for Pregnancy, pgs 14-32

3. Zemjanis, Rectal Examination in the Cow, pgs 1-81

4. Zemjanis, Rectal Examination in the Mare, pgs 91-128

B. Audiovisuals

1. TV-38 Pts I & II, Genital Examination in the Cow

2. TV-396, Pregnancy Examination in the Mare

3. TV-540, Pregnancy Examination in the Mare

4. TV-592, Principles of Ultrasonography in the Mare

5. TV-553, Pregnancy Examination in the Ewe and Sow

C. Other

1. Syllabus

II. Objectives

Given the above resources and following a series of lectures, discussions, demonstrations and hands-on experiences, the student will be able to:

1. Describe the technique of entry of the hand and arm into the rectum of the cow and mare for evaluation of the reproductive organs.

2. Discuss the measures which should be taken to avoid rectal trauma during evaluation of the reproductive organs per rectum.

3. Describe the anatomical landmarks used to locate the genital organs of the cow and mare for evaluation per rectum.

4. Describe the technique for direct and indirect retraction of the uterus of the cow for evaluation by rectal exploration.

5. Adequately identify the palpable structures on the ovaries of the cow on rectal palpation.

6. Identify and correlate the ovarian and uterine findings which can be used to estimate the stage the of estrous cycle on rectal exploration.

 

7. Discuss and demonstrate knowledge of the positive signs of pregnancy in the cow that are detectable upon rectal palpation.

8. Identify and correlate the features of pregnancy diagnosis in the cow by rectal palpation during the following stages of gestation, 30 days, 45 days, 60 days, 3, 4, 5, 6, 7, 8 and 9 months.

9. Discuss the rectal findings which can be used to differentiate a normal gravid uterus from pyometra, mucometra, fetal mummification and fetal maceration.

10. Demonstrate knowledge of the endocrinology of pregnancy in the cow, mare, sow, and ewe.

11. Estimate the age of the bovine and equine fetus based on fetal characteristics.

12. Describe the findings which would be expected on examination (rectal and vaginal) of mares during the following periods:

a. Proestrus c. Diestrus e. Physiological anestrus

b. Estrus d. Pregnancy

13. Identify and correlate the features of pregnancy during the following stages of gestation in the mare: 18-21 days, 30 days, 45 days, 60 days, 90 days, mid trimester (4-7) months) and late gestation to term.

14. Discuss the diagnosis of early pregnancy in the mare based on ultrasonography.

15. Discuss the biological tests used to diagnose pregnancy in the mare and cow.

16. Describe the methods used to diagnose pregnancy in the sow and ewe.

Examination of The Genital Tract of the Cow

 

Before the actual examination is performed, the breeding history of the cow should be reviewed, including the date of the last calving, number of services, and information on any pathologic or disease condition previously affecting the repro- ductive organs.

The tail head and dorsal lumbar area should be examined to determine if the hair is roughed-up or lost as a result of the cow being mounted. When these are present, they suggest that the cow is in heat or has been in heat recently and the examiner should be alert for other evidence. Next, examine the perineum and thighs lateral to the vulva, as well as the underside of the tail at the same level. When pus, mucus or blood are produced in copious amounts, they will be present on and spread by the tail. They provide a useful clue to the alert examiner as to what he/she can expect to find internally.

Next, examine the vulva tuft of hair for the presence of small amounts of metestrual bleeding. This is present 1 to 3 days post-estrus in 90% of first service heifers and only 50% of the time in multiparous animals. The presence or absence of this blood has no relationship to either ovulation or pregnancy. Conception rate in cows bred at this time is only 20% to 30%. Therefore, its principal use is in helping to predict the next estrus. Since heifers have a cycle length of 20 days and cows 21 to 22 days, advise the herdsman to observe the animal for heat in 17 to 18 days as the case may be.

Attention should next be turned to the vulva. The dry, finely wrinkled appearance of the vulva lips of the heifer or cow when not in heat gives way to fewer deeper wrinkles associated with swelling of the vulva lips.

The lips of the vulva are then parted and the mucous membrane examined. When estrogen levels are low, the membrane is pale and wet, but not shiny. Fine arborized vessels (veins) stand out against the pale background. As estrogen levels rise, the increased flow of blood through the fine network of vessels, become visible through the large amount of glistening mucus that adheres to the walls of the vulva during heat, and imparts a pink color to its wall. Examine the clitoral area for pus. When present, it is usually produced locally but may also have originated at a higher level in the tract.

Rectal examination, if properly conducted, represents the only practical method for examination of the genital organs of cows and heifers of breeding age. It is essential that any veterinarian engaged in conducting dairy or beef cattle reproductive herd health programs possess the skills to accurately and quickly evaluate internal genital tract structures by rectal palpation. This includes characterization of ovarian structures, recognition of pregnancy and diagnosis of genital tract pathology. The required skills can only be obtained by educating oneself regarding normal and abnormal findings, by extensive practice and experience in rectal palpation and by constant, careful evaluation of palpation records to assess accuracy. Bovine rectal palpation skills are required to diagnose pregnancy, stage the estrous cycle, diagnose internal genital tract pathology, assess previous treatment of internal genital tract pathology, to transfer fertilized ova between donors and recipients, and to artificially inseminate cows.

 

Restraint

Proper restraint is essential to avoid injury to the examiner. In stanchioned dairy cattle, this is not a problem. On the other hand, a squeeze chute is advisable when working with range cattle.

 

Equipment

Greatest accuracy in bovine rectal palpation can be obtained by wearing a well-fitted sleeve and glove; "one size fits all", disposable plastic sleeves and gloves are generally not acceptable. A non-disposable rubber sleeve with surgical glove attached to fit the palpator is preferred. Because of the risk of exposure to zoonotic disease, cattle should not be palpated without a protective sleeve and glove.

A non-irritating water-soluble lubricant, such as lubrivet, facilitates easier entry into the rectum and further reduces the danger of injury.

 

Entry Into the Rectum

Entry into the rectum is accomplished by forming the fingers into a cone and gently but firmly inserting it into the anus. The presence of the hand in the rectum stimulates the defecation reflex which manifests itself in peristalsis as well as tenesmus. Examination must be discontinued during a peristaltic wave and a wave must never be allowed to pass over the hand. Instead, the wrist should be brought back to the anus but it should not be removed.

Feces must be removed from the rectum for nearly the entire length of the arm without allowing air to enter. Aspiration of air into the rectum causes "ballooning" which absolutely prevents examination. Should the rectum become filled with air, reach forward and gently grasp the first peristaltic ring of mucosa then slowly pull it backward. This will usually stimulate a peristaltic wave which will evacuate the rectum. Removing feces for almost the entire arms length provides surplus rectum which will permit greater ease in performing manipulations of the genital tract.

 

Palpation Techniques

The cervix because of its distinct physical characteristics and relatively constant position, is used as the landmark for locating the internal genital tract. It should be located immediately following emptying of the rectum. The bovine cervix is recognized as a firm, cylindrical, somewhat nodular structure and is usually found lying on the midline on the pelvic floor. However, the cervix may be at or just cranial to the brim of the pelvis even in non-pregnant cows. In most Guernsey and many Shorthorn cows, the cervix is usually found at or beyond the brim of the pelvis. The cervices of Brahma and Brahma-cross cattle are usually quite large, and may normally be a hands width in diameter. In heifers of all breeds, the entire internal genital tract, including the cervix, may normally be found to be displaced laterally. Because of this, the entire pelvic inlet may need to be examined to locate the cervix. Once located, the cervix should be examined for size, form, position, consistency and motility.

If the cervix is found to be freely movable, retraction of the entire genital tract should be attempted. Zemjanis in his book (Diagnostic and Therapeutic Techniques in Animal Reproduction, 2nd Ed. pp 25-27) describes a direct and an indirect method of retraction of the bovine uterus. The indirect method is described as follows: After the cervix is located and found to be freely movable, it is pulled back as far as possible. An attempt is made to bring the uterus up and closer by moving the retracted cervix upward and towards the pelvic wall opposite to the palpating hand. The uterus might be held in this position by placing the thumb underneath the body of the uterus. The next step involves grasping the anterior edge of the broad ligament, which can be accomplished by keeping the thumb in the position described above, turning the hand outward, lowering the bent fingers and hooking the broad ligament from underneath. When this procedure is followed, the broad ligament is grasped between the ovarian end of the uterus and the ovary. It is then lifted dorsally and drawn caudally. The bent fingers are then slid medially, and the uterine horn is gathered in the palm of the hand. The uterine horn is pulled caudally and the fingers are slid further medially until a firm grasp of the ventral inter- cornual ligament is obtained. Further retraction of the uterus is accomplished with gentle traction on the ventral intercornual ligament. This places the uterine horns in the posterior portion of the pelvic cavity, and retraction is complete.

Direct retraction of the bovine uterus is begun by locating the cervix and pulling it as far caudally as possible. The dorsal intercornual ligament is then located and grasped. Slight caudal traction is exerted on the dorsal intercornual ligament to bring up the ventral ligament which is grasped, and the retraction is completed as described for the indirect method.

Once the uterus is retracted, the uterine horns should be palpated, their entire length for size, tone, and consistency. If the horns are empty, they will have a meaty-like consistency.

 

Location of the Ovaries

Bovine ovaries without structures are oval in shape and weigh 5 to 15 grams each. Older cows usually have larger ovaries than younger cows. The ovaries are usually located in the ovarian bursa, a pocket formed by the proper ligament of the ovary and the mesovarium, at the anterior edge of the broad ligament.

The ovaries can usually be located slightly cranial to the pelvic inlet and slightly cranial and lateral to the internal opening of the cervix. They are usually easily located by grasping the anterior edge of the uterus. Left handed palpators can usually locate right ovaries more readily and right handed palpators can usually more readily locate left ovaries. The ovaries are pulled ventrally as the uterusenlarges during gestation or during pathological conditions that cause uterine enlargement.

 

Identifying Ovarian Structures

The experienced palpator should be able to identify and differentiate the following structures on the ovaries of most cows: Ovulation depression (OVD), corpus hemorrhagicum (CH), corpus luteum (CL), follicle (F), and ovarian cyst (C). Before making a diagnosis, the palpator should be sure that the structures palpated are on the ovary.

Although it is possible to detect the ovulation depression in many cows that have just ovulated, it is not unusual for experienced palpators to miss the ovulation depression. The depression feels like a small cavity (usually less than l cm in diameter) with roughened edges. Other tubular tract signs and the history usually indicate the cow was in estrus within the last 24 to 48 hours. The ovarian findings for cows having just ovulated are often recorded as "no significant structures (NSS)".

The corpus hemorrhagicum is a soft structure, which forms in the cavity where the ovulation depression was located. A corpus hemorrhagicum approximately 1 cm in diameter (CH1) is usually present for two to three days following ovulation. This very early corpus hemorrhagicum, if palpable, feels gritty or crepitant. As the corpus hemorrhagicum matures, it progressively increases in diameter to approximately 2 cm (CH2) and then to approximately 3 cm (CH3). A CH2 or CH3 feels muck like a "soft" corpus luteum. The period of time for which the corpus hemorrhagicum is in existence, called the metestrual period, usually last for six to seven days following ovulation.

The mature corpus hemorrhagicum develops into a corpus luteum approximately 3 cm in diameter (CL3). A CL3 increases the size of the ovary, often doubling it. Corpora lutea usually distort the shape of the ovaries in which they are contained. The increased size and distorted form of the ovary with a corpus luteum should be immediately recognized upon grasping the ovary. Most corpora lutea contain a "crown", an extension of the luteal tissue in the form of a prominence above the surface of the body of the corpus luteum. The crown represents the ovulation site. The body of the corpus luteum is usually contained within the ovary and measures approximately 3 cm. in diameter at maturity. Beginning palpators often mistake the crown for the entire corpus luteum. Normal bovine corpora lutea have a "liver-like" consistency, which is evident throughout the entire structure. A distinct line of demarcation between the corpus luteum and the ovarian stroma is usually palpable. Occasionally, a corpus luteum is embedded entirely within the ovary. These corpora lutea do not have protruding crowns and are extremely difficult to identify. As the CL3 ages, it progressively decreases in size to 2 cm in diameter (CL2) and then to approximately 1 cm (CL1.) The CL eventually regresses to a small white fibrotic area, the size of a large pinhead, the corpus albicans (CA).

A prominent difference in ovarian size is usually due to the presence of a corpus luteum on the larger ovary. However, ovarian cysts also often cause a discrepancy in the size of the ovaries. Therefore, the palpator is cautioned against using ovarian size as the sole criterion for diagnosing a corpus luteum.

 

The Follicle

Follicles feel like "blisters" on the ovarian surface. A follicle is smooth and round with a raised surface. It increases from approximately 1 cm in diameter during diestrus to approximately 2.5 cm at the peak of the development. Follicles tend to seem larger whenever they develop on ovaries simultaneously bearing regressing corpora lutea. The margins of follicles are not usually easily discernible. Fluctuation of follicular fluid can usually be detected. It should be remembered that follicles may be present at any time during the estrous cycle or during pregnancy.

 

The Ovarian Cyst

Diagnosis of ovarian cyst is based on the finding of smooth, fluctuating, rounded structures measuring 2.5 cm or larger in diameter, on one or both ovaries. Such findings should be related to other genital tract findings and history. It is possible to confuse cysts with follicles or large diameter, soft corpora hemorrhagica, or soft corpora lutea of pregnancy if one is not very careful and thorough in palpation.

If the structure in question is a follicle, other signs of an impending estrus (for example, regressing corpus luteum, turgid uterus, mucus from the vulva and roughened tail head) should be evident. If the structure is a corpus hemorrhagicum, uterine edema will probably still be evident and perhaps metestrual bleeding as well.

If the structure is a corpus luteum of pregnancy, detection of one of the four positive signs of pregnancy should aid in making a diagnosis.

If the structure is, in fact, a chronic cyst, the uterus will probably be flaccid and a corpus luteum will not likely be present. Although rare, a cow may have an ovarian cyst and a corpus luteumat the same time. Ovarian cyst are occasionally diagnosed in pregnant cows, particularly during the first 90 days of gestation.

 

Staging the Estrous Cycle

The stage of the estrous cycle and the approximate days until the next expected estrus can be estimated fairly accurately by the palpator. Ovarian findings, tubular genital tract findings, physical condition of the cow and reproductive history should be considered prior to predicting the time of the next estrus. Table I gives parameters by which to estimate the stage of the estrous cycle.

 

Table I: Estimation of the Stage of the Ovine Estrous Cycle

Ovarian Uterine Stage Days of Next

Findings Findings Cycle Estrus

__________________________________________________________OVD/CL Tone Metestrus 18-23

CH1 Edema Metestrus 17-20

CH2 Edema Metestrus 15-20

CH3 Flaccid Diestrus 13-18

CL3 Flaccid Diestrus 6-11

CL2/F1.5 Slight Tone Proestrus 1-6

CL1/F2.5 Tone Estrus 0-1

_________________________________________________________

NOTE: Follicles may occur at any time during the estrous cycle. Therefore, estimates of the stage of the estrous cycle should be based on the state of the corpus luteum. The normal bovine estrus cycle is 18 to 24 days in length.

 

Pregnancy Diagnosis in the Cow

 

Endocrinology of Pregnancy in the Cow

The main source of progesterone for the maintenance of pregnancy in the cow is the corpus luteum (CL), the placenta producing only small amounts. The results of ovariectomy and removal of the CL are controversial. Up to about 180 to 200 days of pregnancy removal of the ovary containing the CL, or ablation of the CL either surgically or with the use of prostaglandin F2Alpha (PGF2Alpha), usually results in abortion. However, after this stage until shortly before term (250 days), pregnancy usually continues if the corpus luteum is removed.

Progesterone concentrations during the first 14 days of pregnancy are similar to those of diestrus; therefore those of the nonpregnant cow decline sharply from about the eighteenth day of ovulation. In the pregnant cow, there is normally aslight fall at this stage with a rapid recovery. Thereafter the concentration increases slightly during pregnancy until it starts to decline at about 20-30 days prepartum. Estrogen concentrations during early and midgestation are low (<100 pg/ml); however, towards the end of gestation, in particular after day 250, estrogen concentrations increase to reach peak values 2 to 5 days prepartum of 7ng/ml estrone sulfate and 1.2 ng/ml estrone. These rapidly decline eight hours prepartum to low levels immediately postpartum.

Both FSH and LH concentrations remain low during gestation and show no significant fluctuations. Prolactin is low during pregnancy until just before calving when it increases from basal levels to peak values, until a subsequent decline to basal levels by 30 hours postpartum. Bovine placental lactogen or somatomammotropin is present in the circulation of the dam at about 160 days of gestation, increasing dramatically to maximum concentrations of 1000ng/ml between 200 days and term. Somatomammotropin has actions of both growth of the fetus (perhaps by altering maternal metabolism and making available amino acids and glucose to the fetus) and mammary gland (mammogenesis) during pregnancy.

 

 

 

Pregnancy Diagnosis in the Cow by Rectal Palpation

 

Rectal palpation of the uterus and its contents is the method of choice for pregnancy diagnosis and for estimation of the stage of pregnancy in the cow. The most important factors involved in uterine palpation are proper anatomical orientation and a thorough methodical evaluation of the entire length of the uterine horn.

In the cow, the diagnosis of pregnancy by rectal palpation of the genital tract is based on the detection of the amniotic vesicle, fetal membrane slip, fetus, or placentomes. These are the only positive signs of pregnancy in the cow by rectal palpation. The finding of any one of these signs is sufficient basis for the diagnosis of pregnancy. A diagnosis of pregnancy should not be made unless one of the four positive signs of pregnancy has been detected. Likewise, no cow should be diagnosed as "nonpregnant" unless the uterus has been palpated throughout its entire length. Even in the best managed dairy and beef herds, histories of breeding dates are not always reliable. Therefore every cow examined should be palpated for a possible pregnancy before proceeding with other procedures involving the genital tract.

Retraction of the open or early gravid uterus into the pelvic cavity is essential for quick, accurate evaluation. Retraction can usually be accomplished in open cows and those pregnant less than 90 days. Beyond 90 days retraction becomes more difficult or impossible. By this time, however, placentomes and sometimes fetal parts and membranes can usually be palpated without retraction.

The uterine horns should be palpated their entire length for consistency. If the horns are empty, they will have a meaty-like consistency, if pregnancy exists, fluids will be present in one or both horns. These fluids result in a characteristic fluctuance in one or both uterine horns. A common mistake is that of attempting to slip fetal membranes without first detecting fluctuance. A false slip of the broad ligament or other structure is sometimes obtained, and an open cow is diagnosed as being pregnant.

Palpation of the amniotic vesicle is possible from day 30 to 65 of gestation. Before 30 days it is too small, and after 65 days it is too large and soft to palpate. The vesicle floats freely in the allantoic fluid. To palpate the vesicle, the thumb should be placed on one side of the horn and all four remaining fingers should be placed on the other side of the horn in order to cover large areas of the horn. The vesicle can usually be located in the pregnant horn in the vicinity of greatest fluid enlargement and thinnest uterine wall. It can be trapped in the palm of the hand by working gently up or down the horn. Palpation of the amniotic vesicle should not be used routinely to determine pregnancy because there is danger of rupture of either the vesicle or the fetal heart sac resulting in abortion.

A fetal membrane slip can be performed starting at approx-imately day 30 of gestation. There are no normal uterine fetal attachments in the interplacentomal areas, and in these areas chorioallantoic membrane can be slipped. To do this, a portion of the fluid filled part of the uterine wall is grasped gently and is then allowed to slip between the thumb and fingers (In early pregnancy the entire cross section of the horn should be grasped). The chorioallantois and then the uterine wall slip between the thumb and fingers resulting in a characteristic sensation described as a "blip" if the cow is pregnant. The sensation is similar to that of slipping a thin, taut string crosswise between the fingers. By day 60 to 70 of gestation, it is usually easier to slip the membrane in the nongravid uterine horn.

The fetus cannot be palpated until the amnion becomes flaccid enough to allow manipulation. This usually occurs at about day 65 to 70 of gestation. From this time on palpation of the fetus is valuable for positive diagnosis and determination of the stage of pregnancy.

Placentomes can usually be palpated as early as days 70 to 75. At this time, they can be palpated as ovoid thickened areas in the uterine wall at the intercornual ligament in the horn containing the fetus. The placentomes are larger on the greater curvature of the uterus than near the bifurcation. Palpation of placentomes is particularly helpful at about five to six months of gestation when the fetus is deep in the abdomen and cannot be palpated. The size of the placentomes located at the base of the pregnant horn can be used for estimation of the stage of gestation.

Fremitus of the middle uterine artery is often suggested as an indicator of pregnancy. It can be first recognized at about days 80 to 90 of gestation. Care should be taken not to confuse the internal iliac artery, which is generally attached by fascia to the shaft of the ilium with the uterine artery. The middle uterine artery is in the broad ligament and is freely movable. Individuals in which true fremitus occurs in the absence of pregnancy are rare, so its detection can be considered a positive sign of pregnancy in nearly all cases (Current therapy in Theriogenology 2 - Morrow).

 

Features of Pregnancy Diagnosis at Different Stages of Pregnancy

A. 30 days - Pregnancy diagnoses at this stage are subject to considerable error and are more time consuming. Natural variation in the uterine size and thickness of the walls due to variation in breeds, condition, and age of animal make it more difficult. Accuracy at this stage is directly related to experience.

1. Uterus on floor of pelvis

2. Slight enlargement of one horn with dorsal bulging

3. Thinning of the wall, fluid filled feel

4. Slippage of membrane

5. Embryonic area

6. Corpus luteum in corresponding ovary

7. Confused with non-pregnancy in older cowsand endometritis, possibly malignant lymphoma

a. Differential diagnosis

(1) Unequal size of horns

(2) Slippage of membranes and embryonic area

B. 45 days

1. Uterus still on pelvic floor

2. Greater difference in size of pregnant and non-pregnant horns - 2-2 1/2 inches in diameter

3. Dorsal bulging more pronounced

4. Embryonic area size of small egg

5. Slip membranes

6. Corpus luteum in ovary

7. Not so likely to confuse with endometritis. Pyometra may be a factor to consider (pus in uterus but seal intact.)

C. 60 days

1. Pregnant horn is dropping over brim of pelvis

2. Uterus is 2 1/2-3 inches in diameter

3. Slip membranes in both horns

4. Embryonic area completely fills pregnant horn and not significant

5. Dorsal bulging is gone

6. Uterus feels like a balloon filled with water

7. May be able to bump fetus

8. Corpus luteum present

9. Differential diagnosis

a. Pyometra

(1) Uterus is thick walled with pyometra

(2) Pus can be pushed from one horn to the other

b. Endometritis - uterus is too large

c. If metritis, there would be visible pus on the tail

E. 120 days

1. Fetus well over the brim

2. Palpate legs of the calf, make out definite anatomical form

3. Fetus is 10-12 inches in length

4. Palpate small placentomes

5. Palpate pulse of pregnancy

6. Ovaries may be hard to locate

F. 150 days

1. Uterus is pulled well over brim - cervix at the brim

2. Distinct placentomes - size of ovaries

3. Fetus is well formed. May have to reach for it

4. Good pulse of pregnancy, artery is 1/4 to l/2 inches in diameter

G. 165 to 225 days(5 l/2 - 7 1/2 months)

1. Cervix is right at brim of pelvis

2. Dorsal wall of uterus may be tight and hard to palpate

3. Placentomes are variable in size

4. Good pulse of pregnancy. Artery is l/2 inch in diameter

5. Fetus may not be palpable

6. From 6 months until calving a movement of fetus may be elicited by grasping the feet, legs or nose.

H. 240 days

1. Fetus is back up in range

2. Feel feet

3. Movement is detectable

 

 

Summary of Recommendations Regarding Pregnancy Diagnosis in the Cow by Rectal Palpation

1. Pregnancy determination should be the first step of the genital examination.

2. No animal should be classified as non-pregnant unless the uterus has been retracted and both uterine horns are examined their entire length.

3. No animal should be classified as pregnant unless at least one positive sign of pregnancy is recognized

4. A few animals cannot be adequately examined by even the most experienced - admit this and re-examine at a later date.

5. Do not treat for any reproductive disorder unless you are certain that the animal is non-pregnant.

The average length of gestation in the cow ranges between 270 to 290 days with an average of 284 days. To calculate due date, subtract 3 months from the breeding date, and add 14 days (this is the same as adding 284 days).

 

Example - Bred April 2, 1992 - Due 4-3 = 1, Jan 2 + 14 = Jan 16, 1993

 

 

 

Estimating the Stage of Gestation

Between days 35 and 65, the size of the amnion is the most accurate indication of the stage of gestation.

 

 

Gestation Length (days) Finger Widths cm

35 1/2 0.7

42 1 1.5

48 2 3.5

53 3