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NPR writes in Why Are More Baby Boys Born Than Girls? :

Scientists have found some unexpected clues that could help explain why 51 percent of the babies born in the United States are male.

[...]

So that must mean the skewed sex ratio at birth happens during pregnancy. Looking deeper, the researchers found that in the very first week of pregnancy, more male embryos died, possibly as a result of serious chromosomal abnormalities, which they also documented.

"When that settles out, it looks like there starts to be an excess of female mortality," Orzack says. "And in the third trimester, as has been known for a long time, there is a slight excess of male mortality."

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    $\begingroup$ I suggest that you move this question to SE Medical Sciences, which seems to me both more appropriate and a place you are more likely to get an authoritative answer. $\endgroup$
    – David
    Jul 1, 2021 at 13:05
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    $\begingroup$ If you read the journal article referenced by NPR, the authors give some possible explanations for this phenomenon, including skewed X-inactivation and developmental retardation by the paternal X chromosome. See the second paragraph of the section titled "Overview". $\endgroup$
    – acvill
    Jul 1, 2021 at 17:04
  • $\begingroup$ It's worthwhile to note that the researchers were not able to assemble a uniform and random sample spanning all of pregnancy. Their information came from different sources during different stages of pregnancy and so is not directly comparable. Some of the sources are likely to be biased, e.g. amniocentesis is more likely to be performed if a problem is suspected. $\endgroup$
    – Armand
    Jul 2, 2021 at 14:32

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The statement quoted in the question is hedging, a clear sign that its authors are uncertain of the reasons for the gender imbalance at birth. As with anything in the scientific realm, there are usually more variables than can easily be considered or even filtered.

It used to be that about 106 boys were born for every 100 girls. The numbers of boys are in the decline, a sign of loss in fertility. This is most probably a result of environmental toxins, but may involve other lifestyle factors as well.

WebMD has a page HERE that details some of this (and the CDC concurs). As its author points out, new mothers and young fathers tend to conceive more boys than girls--a clear sign that fertility is involved.

One theory, believed by many but still questioned by others, was promoted by Dr. Valerie Grant, Mr. David Rorvik, and Dr. Landrum Shettles (one article HERE). It involves the physiology of sperm. Biologically speaking, because the female X chromosome is very large, relative to the Y chromosome, "female" sperm are also claimed to be somewhat larger. Their advantage is said to be endurance--being likely to survive in the womb longer than the "male" sperm. But they are thought slower at swimming, particularly given certain environments. Younger couples tend to be more active, and are more likely to have coitus near the time of ovulation, giving the advantage to the "male" sperm. There are other factors involved, however, including the levels of acidity in the birth canal, and even how the mother's body protects and nourishes the sperm while waiting for ovulation.

As to why more females are thought to perish mid-pregnancy, the answer may be as boring as that it simply appears to be so because the mortality of boys at both the early stages and the late stages is higher. Could it be, for example, that boys are more prone to getting strangled by the umbilical cord? There are some distinct gender differences with respect to the development of the umbilical cord and placenta, including increased risks for boys in the latter period of gestation, as noted HERE.

Even though more boys are born, boys have a lower survival rate. They can be more accident prone, more likely to succumb to sickness, or to have more impairments from birth defects. By the early teen years, the male-to-female ratio has usually corrected itself, and by the twenties there are more females in the population. One chart providing a quick overview of this information for US populations can be seen HERE.

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    $\begingroup$ I'd suggest deleting everything after the first paragraph. The 1st paragraph is good, but the rest is not really scientific. What exactly is meant by a "loss in fertility" and why would that affect the sex ratio at birth? There's no verified link to "environmental toxins" whatever they might be, and so forth. $\endgroup$
    – Armand
    Jul 2, 2021 at 14:25
  • $\begingroup$ @Armand If you had followed the links I included, some of your questions might have been answered. Environmental toxins include elemental toxins like lead, cadmium, mercury, arsenic, bismuth, and antimony as well as biotoxins from other sources. They come into our environment in many ways, and there are studies showing how they affect fertility. Toxicologists can quantify the levels of toxins in the body through laboratory tests like a provocative urine test, a hair-mineral analysis, and a newer technology that uses a laser to scan the skin and spectrum analyze it with computer support. $\endgroup$
    – Polyhat
    Jul 2, 2021 at 14:40
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    $\begingroup$ my point is that such "explanations" are so nonspecific as to not be scientific. There is no credible science that I'm aware of indicating that typical exposure levels of any "environmental toxin" affects the sex ratio at birth in the US. Again, the answer says "fertility is involved" - what does that mean? The sperm X or Y content affecting fertilization rate has no credible evidence. The umbilical cord and last paragraph issues seem out of the scope of the question, and the increased mortality rate in the 1st half of pregnancy is real, not an illusion, according to the data shown. $\endgroup$
    – Armand
    Jul 2, 2021 at 15:05

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