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June 11, 2004 (HealthDayNews)
The researchers worked with
women who had either been sterilized or who used an intrauterine
device to prevent pregnancy. They
found the women had more frequent intercourse during the six
most fertile days of the menstrual cycle, peaking at ovulation.
"Among
women who were using effective birth control and not paying
attention to the timing of intercourse, they were much more
likely to have intercourse during the six days in their cycle
when they were fertile," said lead researcher Dr. Allen Wilcox,
a senior investigator with the National Institute of Environmental
Health Sciences.
"Our finding suggests that women who are trying
to get pregnant are having a little help from nature that they
probably aren't aware of. So that's a good thing for couples
who want to get pregnant," he said.
For couples who don't want
to get pregnant, the news is that women who occasionally have
sex without birth control are probably taking a bigger chance
than they realize, Wilcox said.
"It's just at those times when
they are most motivated that they are most likely to conceive.
For whatever reasons -- and we don't yet understand the biological
reasons behind this -- a woman who engages in a single act
of unprotected intercourse is more likely to get pregnant than
was previously believed," he added.
Wilcox's team studied 68
sexually active women over three months -- a total of 171 ovulatory
cycles. Each woman kept a diary of when intercourse occurred
and collected daily urine samples, according to the study that
appears in the June 10 issue of Human Reproduction.
Using the
urine samples to identify the fertile days, the researchers
found intercourse increased 24 percent during ovulation, compared
with the other days of the monthly cycle.
Wilcox speculated
that the increase in the frequency of intercourse in humans
is the same as in other mammals and is due to hormonal changes
that affect a woman's libido. "There are also hormonal shifts
that affect the female's production of pheromones that affect
the libido of males," he said.
"Human biology is constructed
to make couples more interested in intercourse during the very
time they are likely to conceive," Wilcox added.
Dr. Harvey
J. Kliman is a research scientist in obstetrics and gynecology
at Yale University School of Medicine. Commenting on the new
research, he said, "From a personal level, I am sure it's true."
"Having
three children and a wife and watching her for 28 years I know
she is more interested at different times -- there's no question
about it," he said.
Kliman noted that animals function much
the same way. "We are more sophisticated than animals, but
we have many overlaps," he said. "Our systems work to optimize
reproduction."
Dr. Rogerio A. Lobo, director of Reproductive
Endocrinology/Infertility at Columbia-Presbyterian Medical
Center in New York City, added, "I think it is an expected
finding. In fact, I thought this had been done already."
"In
general, mood and libido are improved during an estrogen-positive
phase of the cycle (before ovulation) and is dampened thereafter
-- possibly an effect of progesterone," Lobo said. "This is
nature's way of facilitating reproduction." Added Wilcox:
"The
moral of the story is if you don't want to conceive you really
should use good birth control."
Forbes.com
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Contraception