New research that shows that among women who tried to conceive for over
a year, 58% of the babies born were boys, compared to the normal 51%
birth ratio. Researchers concluded that for each year of trying
to conceive, odds of having a boy increased by an additional 4%
compared to a girl. The study included 5,000 Dutch women, of whom
500 had taken longer than 12 months to become pregnant. All of
the women conceived naturally.
What's the cause for the increased male birth ratio among women who TTC
over a year? The researchers put forth the theory that it has to
do with a woman's cervical mucus (CM), the fluid in the vagina that
sperm must swim through to reach the egg. What's known for sure
is that if the CM is plentiful and slippery, sperm glide through it
towards their goal; but if the CM is sticky and viscous, sperm become
entangled and conception is prevented. The study suggests that
the women who had trouble conceiving had poor quality CM, and that
Y-bearing sperm can more quickly make their way through the viscous CM,
and thus more boys are conceived.
This is going out on a pretty long limb, in my opinion. First of
all, the researchers are assuming that the reason most women didn't
conceive easily was due to poor CM quality. Certainly, this is a
common reason many women don't conceive right away (and easily
corrected), but is it the most common reason? And more to the
point, was it the most common reason among this specific group of
women? Perhaps many of the women just didn't know the right time
to have sex, which is another frequent reason many couples don't get
pregnant right away, because many women just don't recognize the signs
of ovulation.
Secondly, the researchers state the assumption that Y sperm (which
produce boys) swim faster in more viscous cervical mucus. The
research on this topic isn't convincing to me, in that the study cited
is by Dr. Ronald Ericsson, the originator of the Ericsson sex selection
method. The study is from 1973, and uses
a technique later shown to be inaccurate to
prove
that Y sperm can be isolated using the Ericsson method.
Subsequent studies, using accurate testing methods, showed that in
fact, the method did not increase the numbers of Y sperm. However, it
is apparently based on this apparently very flimsy bit of evidence that
the BMJ chose to list this study under the heading "Y sperm may swim
faster than X sperm".
So I'm not really buying this theory that a long time conceiving is equal to poor CM which thus gives Y sperm an advantage.
However, this study does provide some interesting evidence which seems
to be contrary to the Shettles theory -- which, in a nutshell, states
that conditions most favorable to conception will cause a boy to be
more likely, while conditions least favorable for conception will more
likely result in a girl. Yet here we see that when conditions
were unfavorable for conception, for whatever the reason, boys were
significantly more likely.