Length of pregnancy can vary up to five weeks, say scientists
The length of a pregnancy naturally varies between different women by
as much as five weeks, scientists have discovered for the first time.
Although it has always been known that human pregnancies usually last
between 37 and 42 weeks, the variation has previously been attributed to
imperfect methods of estimating due dates.
A woman's due date is calculated as 280 days after the onset of her
last menstruation and is only ever considered a best guess. Only four
percent of women deliver on their due date and only 70 percent within 10
days of it.
However, researchers from the National Institute of Environmental
Health Sciences (NIEHS) were able to pinpoint the moment women became
pregnant by taking daily urine samples - enabling them to isolate the
role that natural variation plays in pregnancy length.
In a group of 125 women, they found the average time from ovulation
to birth was 268 days - 38 weeks and two days - and that the length of
pregnancies varied by as many as 37 days, according to the research,
published in Human Reproduction .
"We were a bit surprised by this finding," said Dr Anne Marie Jukic,
a postdoctoral at the Epidemiology Branch at the NIEHS. "We know that
the length of gestation varies among women, but some part of that
variation has always been attributed to errors in the assignment of
gestational age.
Our measure of length of gestation does not include these sources of
error, and yet there is still five weeks of variability. It's
fascinating."
The length of a pregnancy could be affected by a number of factors.
Older women tend to deliver later, while women who had themselves been
heavier at birth also had longer pregnancies.
The Royal College of Midwives (RCM) said the research reaffirmed
professionals' experience "that every woman is different," and urged
practitioners to be cautious intervening too early in pregnancy, but
said that due dates were still useful.
Mervi Jokinen, practice and standards professional advisor at the RCM
said: "I do not think that this signals the end of midwives and other
health professionals giving a woman a single due date.
It should be explained to the woman that the due date is always an
estimate and as this research and our experience shows, this can vary
widely."
-The Independent
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