Births are gender-biased toward boys in most countries, with close to 105 boys born for every 100 girls born worldwide, according to 2019 data from The World Bank.

But once they’re born, boys seem at least statistically to be more fragile than baby girls in the earlier years.

The comparison of births based on gender is called the sex ratio — and it’s been largely stable for a very long time.

“The first striking point is that in every single country of the world there are more boys born than girls. This has been true for all years for which we have data (back to 1962) in all countries of the world,” according to Our World in Data.

India and China have some of the biggest gaps between male and female births, at 110 males born per 100 females in India and 112 males per 100 females in China, according to The World Bank. Zimbabwe, on the other hand, is closer to parity at 102 males per 100 female newborns. The United States is among those that come in right at the average of 105 boys per 100 girls born.

The Centers for Disease Control and Prevention reported that between 1940 and 2005, an average of almost 92,000 more males than females were born annually in the United States. When the CDC combined all those years, it found mothers older than 40 had a lower likelihood of male births, while those 15 to 19 had the highest likelihood of having males.

But a large 2015 study in the Proceedings of the National Academy of Sciences found that the sex ratio at conception was actually equal. The only way that birth ratios could skew male so consistently, they concluded, was because of gender differences in miscarriages. The study found that, indeed, female mortality is slightly higher over the full course of a pregnancy.

They looked at four stages of a pregnancy and found that which gender is at greatest risk varies, but overall, male-biased sex ratio is the end result: In the first week of pregnancy, more male embryos have chromosomal abnormalities that result in miscarriage. Then, until about 20 weeks gestation, female mortality rises before it levels off mid-pregnancy. At 28-35 weeks, male mortality is higher again. But that long stretch in the middle where more females are lost in utero creates the skew to males at birth.

The male advantage lessens soon after birth. In most countries, infant and child mortality is higher for boys. Experts say infection, prematurity and weaker immune systems account for that.

Boys are more vulnerable to most infections and to birth complications, including acute hepatitis, meningitis, heart anomalies, diarrheal diseases, birth defects, respiratory infections and preterm birth, according to Our World in Data.

Preterm birth is a big complication for baby boys, because though they tend to be heavier, male infants are not as “physiologically mature.” Their lungs, for example, may not work as well. Researchers believe that, among other reasons, female fetuses produce surfactant that helps lung function earlier in development.

It’s also believed that at birth boys have weaker immune systems. Our World in Data says X chromosomes contain more immune-related genes and females have two X chromosomes, while males have one. For the same reason, later, women are more susceptible to autoimmune disorders. The article also notes that testosterone inhibits some important parts of the immune system and males have more testosterone than females.

A 2013 Pew Research Center report said some of the bigger differences in Asia and in the Caucasus can also be blamed on the practice of sex-selective abortion. “The ability to determine fetal sex, along with strong son preferences, accounts in large part for the high shares of boys in many countries in these regions,” the report said.

Other things can also change sex ratios at birth. For example, Pew cited research in the journal Human Reproduction that shows older parents are less likely to have boys. A different study in the Journal of Theoretical Biology finds that during and right after wartime the share of baby boys increases.

The good news is that infant mortality has been falling nearly worldwide for both boys and girls.