Twin pregnancies and the limits of the energetics of gestation and growth hypothesis.

The "Energetics of Gestation and Growth" (EGG) hypothesis proposes that human birth timing and the associated secondary altriciality of human newborns is determined by limits in maternal metabolic capacity. According to this model, labor is triggered when the increasing fetal energy requirements exceed the expectant mother's maximum sustained metabolic scope of 2.0-2.1 × basal metabolic rate (BMR). Because twin pregnancies generally involve higher combined fetal mass than singleton pregnancies, they provide an ideal test of the EGG hypothesis. Here, we use data from one study reporting maternal energy expenditure during twin pregnancy and seven studies that sonographically estimated twin fetal weights. Maternal metabolic scope during twin pregnancies exceeded the suggested metabolic ceiling without triggering earlier birth. Sensitivity analyses indicated that modest increases in total energy expenditure would raise maternal metabolic scope even further, reinforcing that our sample provides conservative estimates. Across studies, twins exhibited fetal growth ratios and energy requirement trajectories (estimated from fetal weight) that were similar to singletons, and variability in both measures increased with gestational age. Fetal energy requirements of two twins at the 50th percentile exceeded the proposed maternal metabolic ceiling in approximately pregnancy week 31, which was on average 5 weeks before birth. Crucially, higher fetal energy requirements were not consistently associated with shorter gestation lengths. These findings contradict the EGG hypothesis and indicate that energetic limits alone cannot explain birth timing or secondary altriciality, even under the substantial energetic load of twin gestations. Instead, the results highlight the adaptability of human maternal energetics.