Women of high-altitude backgrounds, such as indigenous Andean cultures, have increased blood flow to the uterus, and subsequently give birth to larger babies at high altitude, than women without this background. Low birth weights are associated with numerous conditions of poor health and infant mortality.
Healthy birth weight is closely connected to appropriate gestational age (AGA), and those babies born outside this healthy range (both high and low), often suffer from health problems later in life. While the research has not been completely conclusive, some studies have connected both low and high birth weights to obesity, and there has been strong evidence supporting low birth weights increasing risk for adult onset diabetes (type 2). Big babies also appear to be smarter, as increased birth weight has been tied to higher intelligence quotients (IQ). In addition, both low and high birth-weights are more often connected to early life complications for both the baby and mother, including mortality.
High altitude pregnancies, in general, produce lower weight babies, due to lower oxygen levels in the atmosphere, and resulting lower levels of blood and oxygen being delivered to a fetus. Nonetheless, it has now been observed that indigenous high altitude women appear to have developed an evolutionary benefit that allows them to deliver higher levels of oxygen and blood to fetuses, at high altitude, than non-adapted women. The specific mechanism that allows this is within the uterary artery.
The current study was conducted on both women of Andean (high altitude) and European (low altitude) backgrounds, in the country of Bolivia. Pregnant women living in the Andes between 12,000 and 14,000 feet, as well as pregnant women living at sea level, participated in the study. While there were no observed differences in birth-weights between Andean and European women living at sea level, the differences were stark at high altitude.
During pregnancy, the uterine artery dilates in order to increase the flow of blood and nutrients to the fetus. Through analyzing the uterine artery at 20 and 36 weeks into pregnancy, it was observed that nearly twice as much oxygen was being delivered to the fetus (only at high altitude) in pregnant Andean women, than those of European ancestry. In fact, at 20 weeks into pregnancy, total blood flow in Andean women was observed to be 68% greater than European women. While the uterine artery clearly dilated more in Andean women at high altitude, it was still less of a dilation than at sea level for both Andean and European women, where there was no observed difference in blood flow between the two groups. These observations were all made at similar gestational ages between the two groups of women, disconnecting this study some from the common relation between birth weight and gestational age.
The conclusions at high altitude were fairly definitive. First, the more Andean a women was (by tracking her ancestry), the more uterine blood flow she was observed to have. Further, the more uterine blood flow of a women, the larger the birth weight. In study participants at high altitude, Andean women gave birth to babies an average of nine ounces heavier than European women.
Referencing past research, which has shown that Tibetan women of high altitude ancestry give birth to larger babies than Han women, who until recently were at low altitude (and many now have migrated to the Himalayas along with the Tibetans), it is fairly clear that living at high altitude for many generations produced an evolutionary development which benefits fetuses in low oxygen environments. It’s the hope of the current researchers that further understanding of this development will lead to treatments for improving uterine artery flow, both in low and high altitude environments, enabling healthier pregnancies, and future lives, for the children.
Source: Defeat Diabetes Foundation: Guilfoy, Christine. The American Physiological Society press release. May 2009.