Pregnancy length 'varies naturally by up to five weeks'

The length of pregnancy can vary naturally
by as much as five weeks, research
suggests.
The study of 125 women is the first to
calculate gestation by pinpointing the exact
time of conception.
It found age, time to implantation and their
own weight as babies were also linked to
pregnancy length.
An expert said the findings, in the journal
Human Reproduction, challenged whether a
"due date" for women was helpful.
Due dates can be calculated from working out
280 days after the start of the woman's last
menstrual period, or more accurately by
ultrasound.
Yet only 4% of women deliver when predicted
and only 70% within 10 days of their
estimated due date.
Researchers 'surprised'
The research team at the US National Institute
of Environmental Health Sciences measured
hormone concentrations in daily urine samples
taken from women trying to conceive naturally
to determine exactly when ovulation and
implantation of the fertilised egg had
occurred.
They found that the average length from
ovulation to birth was 268 days, just over 38
weeks.
Once they had excluded six premature births,
they found that gestation varied naturally by as
much as 37 days.
Dr Anne Marie Jukic said: "We were a bit
surprised by this finding. We know that 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 study also showed that embryos that took
longer to implant also took longer from
implantation to delivery.
Older women were more likely to have longer
pregnancies and there was also a link between
gestation and a mother's weight when she was
born.
The researchers also found that length of
previous or subsequent pregnancies was
related to the length of the one being studied,
suggesting a consistency about when women
deliver.
But they said it was too early to make any
clinical recommendations.
"I think the best that can be said is that
natural variability may be greater than we have
previously thought and, if that is true,
clinicians may want to keep that in mind when
trying to decide whether to intervene on a
pregnancy," said Dr Jukic.
Dr Virginia Beckett, spokesperson for the Royal
College of Obstetricians and Gynaecologists,
said very little was known about the exact
mechanisms that determine when labour
begins.
"This is a very interesting piece of work and
knowing when is the right time to deliver is a
huge issue."
She added it supports the suggestion that
giving someone a "due date" may not be a
great idea and can make women feel anxious
when they go over.
"It would be better to say, 'You will be
delivered by this time' to take the pressure
off."