In the rhesus monkey placental retention after fetectomy at 121 to 130 days' gestation outlasts the normal duration of pregnancy

Objective: Our aim was to reassess the role of the fetus in the initiation of parturition in nonhuman primates. We tested the effect of the removal of the fetus at 121 to 130 days' gestation on the duration of gestation in pregnant rhesus monkeys. Study Design: Nine monkeys underwent fetectomy with the placenta in situ. Five monkeys underwent surgery without removal of the fetus. Results: In five control monkeys spontaneous vaginal delivery of live fetuses occurred at 163.8 ± 4.6 days' gestation (mean ± SD). In four of nine monkeys that underwent fetectomy the placenta delivered spontaneously at 185,193, 201, and 207 days' gestation. The five remaining monkeys underwent cesarean section at 162, 189, 201, 202, and 219 days' gestation. Duration of placental retention in monkeys that underwent fetectomy (195 ± 16.1 days' gestation) exceeded that in controls (p < 0.05). Plasma progesterone and estradiol concentrations were normal for gestational age at fetectomy, indicating continued placental function. Conclusion: We conclude that the presence of a live fetus plays a significant role in the determination of the duration of normal pregnancy in the rhesus monkey.