Boys Live Dangerously in the Womb

Risky growth strategy can lead to hypertension later in life

Journal: American Journal of Human Biology
Date: April 2010

In the womb, boys have a more dangerous growth strategy than girls. Boys grow more rapidly and invest less in placental growth, putting them at risk of becoming undernourished if maternal nutrients becomes scarce during pregnancy. Since the placenta both nourishes the baby and sustains itself, banking on adequate nutrition from the mother in lieu of a larger placenta is a risky strategy.

In a study involving 2,003 Finnish men and women aged 62 years, scientists set out to understand the role the placenta plays in the long-term risk for developing hypertension. Amazingly, the birth records in Finland contained information about their birthweight, mother’s height, and data on the father’s occupation, grouped into middle and lower class. They also included the weight and width of the placenta. Since the placenta is oval shaped, two measurements were noted: the length and the width. In men, hypertension was associated with a placenta that was wider - in relation to birthweight. The authors set out to discover why.

The study suggests this placental enlargement happens in response to inadequate nutrition during pregnancy. Pregnancies with boys compensate for a lack of immediate nutrition by enlarging the placenta, in an effort to extract more nutrients. It’s the width of the placenta that grows in response to this effort, and it occurs toward the end of pregnancy when a smaller placenta’s ability to transport nutrients would begin to limit fetal growth. In other words, the boys rely on immediate and ongoing nutrition as a growth strategy. Thus, boys have a rapid growth strategy in the womb. They stimulate their mother’s appetite so that she eats more and gains more weight than if she were carrying a girl. When nutritional sources become scarce, the adaptive course of boys is to enlarge the placenta in an effort to extract more nutrients from mother. Girls do not follow the same developmental strategy, and rely more on a mother’s long-term nutritional stores – making it a far less risky approach if there were nutritional shortfalls. The high risk strategy can be fatal for boys. During the Chinese Great Leap Forward famine of 1960-63 more boys died in the womb than did girls.

Compensating for nutritional shortfalls
Compensatory expansion of the placenta has also been studied in sheep. Sheep farmers know that if ewes are put on poor pasture during mid-pregnancy, the placenta will enlarge - presumably to extract more nutrients from the mother. If the ewes are then returned to good pasture in late pregnancy, the enlarged placenta leads to larger lambs. This type of compensatory growth of the placenta also happens in human babies and most often in boys. However, we know from detailed records kept during food shortages in Helsinki before and during the Second World War, as well as the Dutch Famine of 1944-45, that compensatory placental growth usually only occurs if the mother was well nourished at conception and then experiences a subsequent food shortage.

If the food shortage is ongoing, this compensatory placental growth results in a smaller baby with a higher risk for hypertension later in life. This is because the baby had to make developmental “trade-offs” in the womb, with lower priority organs such as the kidneys. During gestation the placenta performs most of the functions that the kidneys would normally perform – thus making the kidneys less of a priority during times of poor nutrition compared to the brain or heart. If this happens, the kidneys take a hit and develop fewer nephrons, becoming permanently shortchanged. The number of nephrons kidneys have is set in the womb, and we now know that this trade-off increases the risk of both hypertension and progressive renal disease.

This study suggests that during development in the womb, boys are more responsive to the mother’s immediate diet than are girls. If the nutritional needs are adequate, boys grow big and weigh more at birth on average than girls. If there are shortfalls, however, their growth strategy puts them at risk for hits to their kidneys and therefore puts them at a high risk for hypertension later in life.

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The OHSU Bob and Charlee Moore Institute for Nutrition & Wellness supports human research that seeks to find the links between maternal stresses, including poor nutrition, and elevated disease risks for babies as they become adolescents and adults. 


Eriksson, J. G., Kajantie, E., Osmond, C., Thornburg, K., & Barker, D. J. (2010). Boys live dangerously in the womb. American journal of human biology : the official journal of the Human Biology Council, 22(3), 330–335.