We have many ways to count the moments of our lives - birthdays, anniversaries, hours in a day, phases of the moon. Perhaps no other time in life makes the countdown by week as important as it is during pregnancy when the 38-week mark is expected to mean a date with the delivery room. Most babies come at their own time, off the mark by a day or two, maybe a week or so, without any cause for concern. News from the US National Institute of Environmental Health Sciences may change the way we count those weeks, though. Researchers at the institute say the new normal may vary by as much as five weeks.

"We were a bit surprised by this finding," said Anne Marie Jukic, Ph.D., the study’s lead author. Her team’s investigation followed 130 pregnancies that were conceived without medical intervention and that involved just one child. Of this group of 130, 125 of them were full-term deliveries that were evaluated for the study.

The team’s findings indicate the median length of pregnancy was 38 weeks, 2 days (268 days) but the range between the shortest pregnancy and the longest pregnancy was 37 days (from approximately 250 days to approximately 286 days). The research team used the date of the last period before pregnancy and hormone levels in the study participants' urine to determine ovulation, implantation, and corpus luteum (CL) rescue pattern. The CL measurement represents the ovaries’ hormonal response to implantation of the egg in the uterus.

Some contributing factors to longer pregnancies included:

  • Longer time between conception and implantation signals longer pregnancy
  • A rapid rise in progesterone (measured by CL rescue pattern) indicated a pregnancy lasting about 12 days longer than when progesterone levels rose more slowly
  • Older mothers
  • Longer previous pregnancies
  • Mothers heavier at birth

Factors that did not seem to influence longer pregnancies included alcohol intake, body mass index (BMI), sex of child, and previous pregnancies, if any.

Study participants who experienced longer pregnancies had experienced long previous pregnancies, indicating the length of pregnancy remains about the same from woman to woman, not pregnancy to pregnancy. Also indicated was the importance of the biological events in the first two weeks of pregnancy as predictors of the length of the pregnancy. Delivery dates have never been better than educated guesstimates since it's impossible to knowthe date of conception and implantation.

Source: A.M. Jukic, D.D. Baird, C.R. Weinberg, D.R. McConnaughey, and A.J. Wilcox. Length of Human Pregnancy and Contributors to Its Natural Variation. Human Reproduction. First published online August 6, 2013 doi:10.1093/humrep/det297