Gestation period varies widely in the mammalian world, with some species developing twice as fast as others in the womb. This is largely because of differences in the arrangement of fetal and maternal tissues in the placenta.

Isabella Capellini at Durham University, UK, and her team analysed data from previous studies on neonatal brain mass, body and litter size, and maternal placental morphology from 109 mammalian species. They discovered that animals with placentas where fetal and maternal tissues interlock the most — creating a greater surface area over which nutrients can flow — gestate in less than half the time taken by animals that have placentas with a minimal surface area for nutrient exchange.

Am. Nat. doi:10.1086/657435 (2010)