382 ULTRASONOGRAPHIC EVALUATION OF AMNIOTIC SAC SIZE DURING EARLY PREGNANCY IN COWS

Abstract

In ruminants, the embryonic vesicle is composed of the yolk and the allantoic and amniotic sacs. In cattle, the allantoic sac represents the largest fluid accumulation in the uterus from early pregnancy onward, but is irregular in shape as it conforms to the uterine horns. It is, therefore, difficult to ultrasonographically measure the allantoic compartment. Conversely, the amniotic sac remains ellipsoidal in shape and can be evaluated more easily. The aim of this study was to characterize the development of the amniotic sac in relation to embryo development. Thirteen multiparous, nonlactating Aberdeen Angus cows were examined weekly from 30 days to 72 days of gestation. Epidural anesthesia was induced (6 mL of 2% lidocaine) to obtain light relaxation of the rectum and prevent straining. Measurements were taken by ultrasonography (Toshiba Nemio 20, Tokyo, Japan) using a 5–10 MHz intraoperative finger probe from 30 days to 58 days of pregnancy, and a 3–6 MHz linear-array probe thereafter. Length (D1) was registered in a longitudinal section of the amniotic sac, whereas width (D2) and height (D3) were measured in a cross section. Volume (cm3) was calculated with the formula 4 ÷ 3 × Pi × (D1 ÷ 2) × (D2 ÷ 2) × (D3 ÷ 2) ÷ 1000 and mean amniotic diameter (MAD) was calculated as the average of the three measurements (Hellman et al. 1969 Obstet. Gynecol. 103, 789). Crown rump length (CRL) was measured in a longitudinal section of the embryo when it was in a neutral position. Results show that volume, MAD, and the CRL increase progressively during the first trimester (Table 1). In the first 50 days of gestation, the difference (MAD-CRL) changed from a negative to a positive value, probably as a reflection of increasing embryonic production of amniotic fluid (Wallenburg et al. 1988 Med. 5, 191). From 58 days of gestation onward, the difference became progressively more negative due to a faster relative growth rate of the embryo than of the amniotic sac. To conclude, the present table may be useful in distinguishing between normal and abnormal early pregnancy, including embryonic growth retardation and hydrops amnios (Wintour 1986 et al. Aust. Vet. J. 216, 221). Table 1. Mean amniotic diameter (MAD), crown rump length (CRL), and the difference (MAD-CRL) during pregnancy in cattle (mean ± SD; n = 13)