Abstract Gestation periods in mammals are generally in line with scaling laws. There are several exceptions in which species of relatively similar size and degree of neonatal maturation show a significant difference in gestation length. For example the giraffids have a very long gestation period, compared to bovids of similar size. By using published data about the placental surface area at term, we show that in ungulates this surface area is more tightly correlated to the average fetal growth rate than to fetal and placenta weight. These data suggest that, within one type of placenta, gestation length and placental surface area are associated parameters.