Reproductive aspects of New World monkeys are reviewed in an evolutionary context. Molecular phylogenies clearly show that they are monophyletic. However, they also show that clawed New World monkeys are not sister to all other platyrrhines. Instead, the first divergence separated titis, sakis and uakaris from all others. Some resulting cladistic classifications confusingly include clawed New World monkeys in Cebidae. Yet a classical grade-based classification can maintain a division between Cebidae and Callitrichidae (which are specialized evolutionary dwarfs). In molecular trees, Callimico also consistently nests within callitrichids, being more closely related to marmosets than tamarins. Ovarian cycles and gestation are reviewed, notably in comparison to the sister group (Old World monkeys, apes and humans). New World monkeys similarly have invasive placentation and menstruation (albeit weakly evident in callitrichids). New World monkeys are unusual among primates because ovarian cycle lengths are often reduced. Cebids have single births; but marmosets and tamarins show secondary evolution of multiple births, with a highly unusual shared placental circulation resulting in chimaerism. The lag phase in early gestation also shows secondary extension among callitrichids, including Callimico. Multiple pregnancies were seemingly secondarily suppressed during evolution of Callimico from an ancestor shared with marmosets.