It won’t surprise anyone to hear that boys and girls are very different creatures, but to find that these differences are noticeable at the earliest stages of development has surprised even scientists. Differences in growth patterns are seen from the moment of fertilization and have long-term implications for adult health.

Upon fertilization, an egg is already carrying the entire genetic blueprint of a new individual, including the sex, which was determined by whether the fertilizing sperm carried an x or a y chromosome. During its week-long travel to the uterus the fertilized egg, now a single cell embryo, is dividing again and again. It turns out that male embryos wait less time between cell divisions compared to female embryos. This means male embryos grow faster and consume more nutrients even before burrowing into the lining of the mother’s womb to trigger a pregnancy.

Boys appear to be “hungrier” during their fetal life too. This makes sense considering their growth plan is to grow as fast as possible. However, this strategy makes them vulnerable to undernourishment and later life health problems. A fetus can grow only as fast as allowed by its ability to get “food” from its mother. Remember that the fetus depends entirely on the placenta for nutrition. The placenta captures nutrients from the mother and transfers them directly into the fetal blood.

Nutrients are difficult to acquire when you are a fetus and males don’t make it easy on themselves because, contrary to logic, they make the smallest placenta possible. Why waste precious nutrients by investing in building a placenta when your strategy is simply to grow as fast as possible? To compensate, boys use two clever strategies to grow rapidly with a smaller placenta. First, they make their placenta more efficient. Boys are able to grow faster with less placental tissue than girls because male placentas are better able to take up nutrients. Second, boys stimulate their mothers to eat more. Somehow, they are able to increase the appetites of their mothers. Mothers carrying boys consume more protein, carbohydrates and especially fats and thus they gain more weight than mothers carrying girls.

This go-for-broke strategy of males is presumably a feature that leads to large, physically able men. However, the male growth strategy is fraught with danger because it depends on the constant food intake of mothers. In times of famine or placental difficulties, boys in the womb die more easily and more often. In times of disaster, fewer boys are born alive than girls. This phenomenon has been seen following wars, famines and natural disasters. Stress experienced while in the womb leaves boys more vulnerable to detrimental outcomes during development and after birth. Among babies born pre-term, more boys die than girls. Boys who are born small and grow rapidly in early childhood live shorter lives. 

Girls on the other hand have a slow growth-large placenta strategy that protects them from possible nutrient shortages in mothers. They are born on average, shorter and lighter than boys. Girls in the womb grow more slowly and depend less on their mother’s day-to-day food consumption and more on the stores of nutrients she has built up over her adolescence and adulthood. This makes them better able to sustain changes to a mother’s diet.

Rapid growth has its price and that price is paid more often by boys in the womb. The ultimate manifestation of their dangerous growth strategy may be that men have higher blood pressures and shorter lives than women. During development, boys’ fast-growth strategy prioritizes growth of important organs like the brain. Organs like the kidney, whose function is performed by the mother until after birth may be shortchanged, leading to a higher likelihood of developing hypertension and heart disease later in life, and thus resulting in a shorter lifespan.

Both boy and girl babies will benefit from mothers who have healthy bodies when they become pregnant and who eat a healthy diet while their offspring are growing in the womb.