Through the years, it has been understood that male babies need more nutrition from their breast milk than female babies. The breast milk that mothers produce when they have male babies has a higher energy content that that of mothers who give birth to females. The body’s response to gender is completely automatic, and this variation in lactation content has been proven in many studies. However, a recent study that examined the difference in breast size in these two cases showed surprising results. Since the nutritional content is higher for male babies, one would assume that the breast size would be larger in that case. However, the study proved the opposite. If you give birth to a baby girl, your breast size will increase more during your pregnancy than if you were giving birth to a baby boy.
The reason behind this difference is completely evolutionary. Before humans could protect themselves in modern homes and in modern society, mothers had to be very careful with their children. They would need to protect them from many predators and would struggle to find enough food, much like wild animals. When a mother had a male baby, he would be a little bit better suited to defend himself from predators, and stronger to make it through bouts of hunger. Female babies on the other hand were weaker and required extra protection. For that reason, the mothers’ body would change to attract her male partner more through pregnancy so that he’d stick around and help raise the baby girl. In other words, your breast size will increase if you have a baby girl so that your husband will want to stay with you and help raise the girl- or so the theory states.
Now that we have modern technology, medicine and homes, protecting a baby girl and raising her to be healthy is just as easy as doing so with a baby boy. However, our bodies still react in the same way, since it is a natural part of our progression as human beings. Larger breasts were meant to keep the pregnant women attractive so that their mate wouldn’t leave them. In fact, the popularity of breast implants in today’s society show that this principle lives on in a different way outside of pregnancy. Pregnancy is incredible on its own, and when you add the changes built in from hundreds of years of evolution, it only becomes more so.
Source: Andrzej Galbarczyk: Unexpected Changes In Maternal Breast Size During Pregnancy In Relation To Infant Sex. American Journal of Human Biology Volume 23 Issue 4 pp. 560-562 May 2011