Males, females conceived evenly, but females have higher mortality rate during pregnancy

Reuters/Arko Datta

Reuters/Arko Datta

The sex ratio at conception is 50-50 between males and females, but more males are born throughout the world because of a higher mortality rate for female fetuses, according to a new study that focused on the sex ratio from conception to birth.

“It’s important to study male-female differences in the womb
because they underlie, in part, the profound differences we see
between males and females at birth and thereafter,”

biologist Steven Orzack at the Fresh Pond Research Institute in
Cambridge, Massachusetts said, according to HealthDay.

Gene expression starts at the first cell ‒ or immediately after
fertilization ‒ and the first linkage to the X- or Y-chromosomes
occurs at the eight-cell stage, meaning that gender is assigned
to the fetus almost at conception. In general, around 105 boys
are born for every 100 girls worldwide, a natural phenomenon that
was first studied in the late 17th century, and that is not due
to selective abortions of female fetuses in some regions.

“Our results indicate that the sex ratio at conception is
unbiased, the proportion of males increases during the first
trimester, and total female mortality during pregnancy exceeds
total male mortality,”
the authors, led by Orzack, wrote
about the significance of
their study
.

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The data disproves the claim that conception is more male-biased
than the birth sex ratio, an assertion that “appears often in
textbooks and in scientific literature, usually with little or no
description of evidence,”
the researchers wrote, noting that
previous estimates of the primary sex ratio “have no
meaningful basis in data from the time of conception (or within
at least a month of it.”

To estimate the sex ratio at conception, they examined data on
nearly 140,000 embryos that had been created in fertility
clinics, along with almost 900-thousand samples from fetal
screening tests like amniocentesis, and 30 million records from
abortions, miscarriages and live births. The embryos ‒ aged three
to six days old ‒ that had been routinely screened at fertility
clinics in the United States and Canada for genetic problems.

In the US, 51 percent of the babies born are male.

“Our dataset is the most comprehensive and largest ever
assembled to estimate the sex ratio at conception and the sex
ratio trajectory and is the first, to our knowledge, to include
all of these types of data,”
the authors wrote.

Orzack and his fellow researchers from Harvard, Oxford and
Genzyme Genetics found that the sex breakdown was virtually even
at the point of conception.

“The best estimate we have is that it’s even-steven ‒ 50
percent males [and] 50 percent females,”
Orzack told NPR.

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The researchers also found that the sex ratio among abnormal
embryos is male-biased, and the sex ratio among normal embryos is
female-biased, which may be associated with the abnormal or
normal state of the sex chromosomes and of chromosomes 15 and 17.

The sex ratio decreases in the first week or so after conception,
meaning there is an excess of male mortality. The ratio then
increases for at least 10-15 weeks as more female fetuses die,
and then levels off after approximately the 20th week of
pregnancy, the authors found. The sex ratio finally declines
slowly from week 28 to week 35 due to another excess of male
fetuses dying.

“The unbiased sex ratio at conception, the increase in the
sex ratio during the first trimester, and total mortality during
pregnancy being greater for females are fundamental insights into
early human development,”
the researchers wrote.

In the future, Orzack said the group would like to figure out why
more females are dying than males during the early stages of
pregnancy and why the ratio switches later on, with more males
dying during the latter stages of gestation, according to
HealthDay.

The paper was published in the latest edition of the Proceedings
of the National Academy of Sciences of the United States, a
scientific journal.

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