Timing of the luteal-placental shift is delayed with additional fetuses in litter-bearing callitrichid monkeys, Saguinus oedipus and Callithrix jacchus

•In primates, including humans, control of pregnancy shifts from the corpus luteum to the placenta typically during the first trimester.•Luteal control is indicated by chorionic gonadotropin production. The shift to placental control is accompanied by a rise in the production of glucocorticoids, which are essential for fetal maturation.•Marmosets and tamarins are monkeys that produce multiple multizygotic fetuses.•In the marmoset and tamarin, more fetuses are associated with a longer period of elevated production of chorionic gonadotropin to maintain the corpora lutea.•In larger litters, the longer period of luteal control is followed by a later rise in glucocorticoids which are essential for fetal maturation.•When there are more fetuses, the corpora lutea are in control longer to allow the placenta more time to mature to support the growth of multiple fetuses.