Measurement of fetal biparietal diameter in owl monkeys (Aotus nancymaae)

Abstract

Owl monkeys are New World primates frequently used in biomedical research. Despite the historical difficulty of breeding owl monkeys in captivity, several productive owl monkey breeding colonies exist currently. The animals in the colony we describe here are not timed-pregnant, and determination of gestational age is an important factor in prenatal care. Gestational age of human fetuses is often determined by using transabdominal measurements of fetal biparietal diameter. The purpose of this study was to correlate biparietal diameter measurements with gestational age in owl monkeys. We found that biparietal diameter can be used to accurately predict gestational age in owl monkeys.

Figure 1.

Ultrasound images of owl monkey fetuses during the second half of pregnancy. (A) Typical image showing the fetal head in a position for measuring gestational biparietal diameter (BPD). f, falx; t, thalamus. (B) Thorax. a, appendage; h, heart; p, placenta. (C) Abdomen. a, appendage; *, spine; s, stomach. Ultrasonography can be used to reliably assess fetal viability and morphology. During a typical exam, the following fetal structures are observed and evaluated: skull, heart, limbs, spine, stomach, and umbilical artery and vein (not shown). The placenta is evaluated for appropriate placement and echogenicity. BPD and fetal heart rate typically are measured also.

Figure 2.

A graph of gestational biparietal diameter (BPD) of Aotus nancymaae fetuses. Measurements were made across the fetal head, with clear visualization of the thalamus, falx, and third ventricle at the level of the cavum septum pellucidi. BPD was determined at the widest point of the head circumference. Data were collected at random intervals throughout the pregnancies of 27 dams. Dams were not time-mated but were members of a breeding colony. Days prior to birth was calculated after birth by counting back to the date of the ultrasound exam and subtracted from the assumed term length of 133 d to obtain gestational age (that is, days of gestation). Linear regression was calculated by using the Marquardt–Levenberg (order 2) algorithm, with r² = 0.94. Gestational age can be calculated as 12.11 + (43.66 × BPD) + [(BPD − 2.07)² × 4.21].

Figure 3.

A scatter plot of fetal heart rate (FHR) as correlated to trimester. Although it varies considerably among animals, FHR is greater than 150 bpm in viable fetuses.