Variability Length Of Pregnancy
August 7, 2013

Variety Of Factors Lead To Suprising Variability In Pregnancy Length

Brett Smith for - Your Universe Online

Using a new technique that tracks the growth of a baby from ovulation to birth, researchers from National Institute of Environmental Health Sciences (NIEHS) found a significant discrepancy between the calculations used to determine due date and the average length of a human pregnancy.

They also found that the length of pregnancy varies by around five weeks, and that this variability is related to a mixture of factors. Women are typically given a due date 280 days after the onset of their last menstrual period. However, only about 4 percent of women deliver at the 280-day mark.

"We found that the average time from ovulation to birth was 268 days - 38 weeks and two days," said Anne Marie Jukic, a researcher on the new study and epidemiologist at the NIEHS, a branch of the National Institutes for Health (NIH). "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," Jukic said. "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," she added. "It's fascinating."

In the study, which was published in the journal Human Reproduction, the research team culled data from daily urine samples taken in a 1980s study that followed 130 singleton (non-twin) pregnancies from natural conception to birth. The volunteers reportedly had no fertility problems and had stopped contraceptive measures to become pregnant. They were also healthy and less likely to smoke or be obese. The participants also kept daily diaries and collected daily morning urine samples for six months or until the end of the eighth week if they became pregnant. These urine samples were analyzed for pregnancy-associated hormones: hCG (human chorionic gonadotropin), estrone-3-glucoronide and pregnanediol-3-glucoronide.

"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," Jukic explained.

In addition to finding varying lengths of pregnancy, the study also revealed that embryos that took longer to implant also had longer time from implantation to delivery. Additionally, pregnancies that showed a later rise in the pregnancy hormone progesterone averaged 12 days shorter than pregnancies with an earlier increase.

"I am intrigued by the observation that events that occur very early in pregnancy, weeks before a woman even knows she is pregnant, are related to the timing of birth, which occurs months later," Jukic said. "I think this suggests that events in early pregnancy may provide a novel pathway for investigating birth outcomes."

The study found that older women tended to deliver later, with each year adding about one day to the pregnancy. Moreover, women who were heavier at their own birth had longer gestations, with each 3.5 ounces of the mother's birth weight corresponding to about one-day longer pregnancy.

"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," Jukic concluded.