The Science of Having a Boy or a Girl

Most parents-to-be want a healthy baby, but many also wish for a particular gender. You have probably heard rumors for tipping the odds in favor of one sex or the other, but are any of them true? Explore the science of having a boy or a girl.

The Father’s Sperm Determines a Child’s Gender

No matter what other factors may be at play, one thing is certain—the sex chromosome of the sperm that fertilizes the egg determines the baby’s gender at birth. If an X chromosome combines with the mother’s X chromosome, you get a girl (XX). And if a Y chromosome reaches the egg first, you get a boy (XY).

Factors that May Influence the Sex of a Child

Just about everyone has a nearly 50/50 chance of conceiving a boy or a girl. Having a family of all boys or all girls is almost always due to pure chance. Still, specific factors may slightly influence the odds that an X or Y chromosome sperm will fertilize the egg.

  • The ovulation cycle: In the 1960s, the best-selling book, How to Choose the Sex of Your Baby, argued that timing sex close to ovulation allows fast-swimming Y sperm to reach the egg sooner, resulting in a male baby. And since Y sperm die quicker, intercourse far away from ovulation maximizes the chance of X sperm achieving fertilization, resulting in a female baby. However, this argument has been disputed over the years.
  • Wartime: Some research has found a small correlation between higher male birth rates and times of war and conflict. This is an interesting find, given that male mortality rates are high during war.
  • Stress: Other studies show that extreme stress can lead to female conceptions, which may be related to the fragility of Y sperm or the hormonal changes that tend to favor X sperm when the going gets tough.
  • Wealth: A study of billionaires suggests that men who inherit their money are more likely to have sons than self-made billionaires (and the general population), suggesting that wealth without stress leads to sons.
  • Mother’s diet: Some argue that eating cereal preconception induces male births, while others say low-salt, high-calcium diets favor females.
  • The father’s family: A study of family trees found that fathers inherit a tendency to have more sons or daughters from their parents. Therefore, a man with many brothers is more likely to have sons, and a man with many sisters is more likely to have daughters.

With so many factors at play, most of which aren’t within your control, the odds end up being about the same as flipping a coin. And chances are, the moment you hold your newborn, it won’t matter to you whether it’s a boy or a girl.

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