The duration of a human pregnancy It can naturally vary up to five weeks, according to research published in this Wednesday’s issue of the journal ‘human reproduction‘. Normally, women are given an estimated due date of 280 days after the start of their last menstrual period, but only 4 percent of women deliver after that time and only 70 percent do so at 10 days from your due date, even if the date is calculated with the help of ultrasound.
American researchers have been able to locate the exact point at which a woman ovulates and a fertilized embryo implants in the uterus during a naturally conceived pregnancy and have followed the pregnancy until delivery. Using this information, they have been able to calculate the length of 125 pregnancies.
“We found that the median time from ovulation to birth was 268 daysthat is to say, 38 weeks and two days“said Dr. Anne Marie Jukic, a postdoctoral fellow in the Epidemiology Service at the National Institute of Environmental Health Sciences in Durham, part of the National Institutes of Health. “However, even after having excluding six preterm births, they found that the length of pregnancies varied by up to 37 days.
“We were a bit surprised by this finding. We know that gestation length varies between women, but some of that variation has always been attributed to errors in the gestational age assignment. Our measure of gestation length does not include these sources of error, and yet there is still five weeks of variability. It’s fascinating,” says this expert.
The possibility that the duration of pregnancies may vary naturally has been little investigated, as it is impossible to differentiate between miscalculations and natural variability without being able to correctly measure the gestational age of a developing fetus.
Previous studies from the 1970s and 1980s had used the slight increase in a woman’s body temperature upon awakening as a way to detect when ovulation occurred, an inaccurate measurement that cannot be used to detect when the embryo was implanted in the uterus.
In the current study, the researchers took information from daily urine samples collected by women who participated in an earlier study, the North Carolina Early Pregnancy Survey, which took place between 1982 and 1985 and analyzed 130 singleton pregnancies. without help, from conception to birth.
The women had stopped taking contraceptives in order to become pregnant, were healthy with no known fertility problems, and were also less likely to smoke or be obese. Study participants completed daily logs and submitted urine samples first thing in the morning for six months or until the end of the eighth week if they became pregnant.
Urine samples were analyzed for three hormones related to early pregnancy: hCG (human chorionic gonadotropin), estrone-3-glucuronide and pregnandiol-3-glucuronide. He ovulation day it was identified by the fall in the ratio between the hormones estrogen and progesterone, and embryo implantation was identified as the first day of a sustained increase in hCG levels.
“Since the embryo secretes hCG and mothers generally have little or no hCG in their urine when they are not pregnant, the first hCG surge was used to indicate implantation,” Jukic explained. In 2010, the scientists contacted the women during the present study to obtain information about their labor and whether an induction or caesarean section had occurred, all of which information was available on 125 pregnancies after excluding those affected by exposure to the diethylstilbestrol, an endocrine disruptor known to reduce pregnancy.
In addition to the variation in the gestation lengththe study found that embryos that took longer to implant also had a longer time from implantation to delivery, and that pregnancies with late progesterone surge were significantly shorter, with a median of 12 days. of pregnancy less.
Jukic said: “I am intrigued by the observation that events that occur very early in pregnancy, weeks before a woman knows she is pregnant, are related to the time of birth, which occurs months later. I think this suggests that events in early pregnancy may provide a new avenue for investigating birth outcomes.”
Other factors that seem to influence the duration of pregnancy They include: older women give birth later, averaging about one more day of pregnancy for every year of age; women who had been heavier at birth had longer pregnancies, with every 100 grams in the mother’s own birth weight corresponding to about one more day of pregnancy, and whether a woman had longer pregnancies earlier or later to gestation, the new pregnancy was probably longer.