As if expectant mothers don't already have enough on their plates, a new study shows human pregnancy can naturally vary as much a five weeks.
The findings were published online Wednesday in the journal Human Reproduction.
Doctors calculate the baby's first due date by estimating 280 days after the onset of a woman's last menstrual period, and later use the help of an ultrasound to give an update. However, only four percent of women deliver at 280 days and only 70 percent deliver within 10 days of their estimated due date.
For the first time, researchers have been able to "pinpoint the precise point at which a woman ovulates and a fertilized embryo implants in the womb during a naturally conceived pregnancy, and follow the pregnancy through to delivery," according to a news release.
With this new information, scientists were able to calculate the length of 125 pregnancies in the study.
"We found that the average time from ovulation to birth was 268 days - 38 weeks and two days," said Dr Anne Marie Jukic, a postdoctoral fellow in the Epidemiology Branch at the National Institute of Environmental Health Sciences (Durham, USA), part of the National Institutes for Health, in a news release. "However, even after we had excluded six pre-term births, we found that the length of the pregnancies varied by as much as 37 days.
"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."
According to the news release, researchers retrieve data using daily urine samples collected by volunteers in an earlier study, the North Carolina Early Pregnancy Study, which took place between 1982-1985:
"[Scientists] followed 130 singleton pregnancies from unassisted conception through to birth. The women had discontinued contraception in order to become pregnant; they were healthy, with no known fertility problems and they were also less likely to smoke or be obese. The women completed daily diaries and collected daily first-morning urine samples for six months or until the end of the eighth week if they became pregnant."
Urine samples from the expectant mothers were analyzed for three hormones connected with the onset of pregnancy: hCG (human chorionic gonadotropin), estrone-3-glucoronide and pregnanediol-3-glucoronide. According to researchers, "the day of ovulation was identified by the drop in the ratio between the hormones oestrogen and progesterone. Embryo implantation was identified as the first day of a sustained rise in levels of hCG."
"Since the embryo secretes hCG, and mothers generally have little to no hCG in their urine when they are not pregnant, we used the earliest increase in hCG to indicate implantation," explained Dr Jukic.
To read the full news release, click here.
