Thick Cervical Mucus Promotes Sons
A recent study suggests that women who take longer to become pregnant have a higher chance of bearing sons when compared to women who conceive right away. The Dutch study found that in a group of 500 women who took longer than a year to become pregnant, 58% had baby boys. In a much larger group of 4800 women who became pregnant before a year had passed, the rate of those who gave birth to baby boys was only 51%. Perhaps more striking was the finding that for each additional year that passes as a couple attempts to conceive without treatment, the probability of conceiving a baby boy goes up almost 4%. This figure was determined after researchers factored in smoking, alcohol consumption, and irregular menstrual cycles.
Luc Smits, an epidemiologist at Maastricht University in the Netherlands, was the lead researcher on this project. Smits believes that these results support the notion that male sperm are faster swimmers than those sperm containing the female X chromosome. The male sperm have less difficulty navigating their way through thick cervical mucus. The epidemiologist says that earlier studies have found that the viscosity of cervical mucus is greater in some women and that the thicker the mucus, the longer it takes those women to become pregnant.
New Zealand-based University of Auckland's Valerie Grant finds the research conducted by Smit's team an "elegant piece of research." Grant has specialized in determining the mother's role in the ratio of boy to girl offspring in humans. There are animals that have the ability to influence the sex of their babies based on the quality of their mates and surrounding conditions. Experts in evolutionary biology posit that this ability helps them leave a larger number of surviving descendants.
According to this idea, there is a theory that animals with lower status undergo greater deprivation and have more female babies. Giving birth to and raising a son requires a greater investment of energy. Female offspring therefore have better survival odds during stressful times. "These findings are not in agreement with the theory. But they also do not contradict this theory as less fertile women are not necessarily less healthy," says Smits. "We do not have an evolutionary explanation for this."
Grant, however, doesn't buy Smits' explanation that conceiving a boy after a lengthy try at conception means that the Y chromosome sperm are better at getting through viscous mucus. The New Zealand expert posits an alternative theory that poor quality mucus is usually connected to hormonal issues which will then increase the rate for miscarriage. In Grant's research, the influence wielded on the sex of a child is only derived from the mother and not from the father. This is in line with classical evolutionary biology theory.
Grant explains that males have a higher mortality rate from the time of conception and on. In order to achieve a certain number of male babies during stressful times, a mother may change the ratio so that more males are born. Whatever mechanism is behind this changing ration is meant to increase the fitness of the offspring.