Let Me Count the Days 'til the Baby

Normal pregnancies vary in length from 35 to 40 weeks

/ Author:  / Reviewed by: Robert Carlson, M.D Beth Bolt, RPh

(RxWiki News) Common wisdom holds that a pregnancy lasts nine months. Those with experience of actual pregnancies, however, know that the length of time can vary.

A recent study found that normal, healthy pregnancies can vary by as much as 37 days.

The variation from 35 to 40 weeks in normal full-term pregnancies was found after taking into account women who gave birth especially early or late.

The pregnancies were measured based on the women's hormones, starting from conception and tracking the progress of the pregnancy.

"Attend all prenatal appointments."

This study, led by A.M. Jukic, of the National Institute of Environmental Health Sciences in Durham, North Carolina, tried to determine the variation in women's pregnancy lengths.

A typical pregnancy without complications is thought to be 38 to 40 weeks, counting from the first day of a woman's last menstrual period.

The researchers tracked 125 pregnant women, with an average age of 29, from conception through their children's births. Nearly all of the women were white, nonsmokers and most had a college degree.

All of the women conceived naturally, without in vitro fertilization, fertility medications or other assistance. About half the women already had children and 80 percent of them had a normal (healthy) weight.

The researchers used measurements of the women's hormones in their urine to track the progress of the pregnancy, including their ovulation (the release of their egg) and the implantation of the fertilized egg in a woman's uterus.

In measuring the length of each woman's pregnancy, the researchers took into account cesarean sections (C-sections), labor inductions and other interventions or complications that might shift the apparent length of the pregnancy.

A labor induction means a woman received medication to cause her to begin having labor.

Six of the women had preterm births and four had their babies post-term. Post-term means at least two weeks past the due date, and preterm means more than three weeks before the due date.

In determining the average length of a normal pregnancy, the researchers excluded the women who had preterm births or women with pregnancy-related complications or medical conditions.

The researchers found that the average length of a normal pregnancy based on this small group was 268 days, or 38 weeks and 2 days.

The variation among the women's pregnancy lengths for full-term pregnancies was 37 days. This means the difference between the shortest normal full-term pregnancy and the longest normal full-term pregnancy among the women was 37 days.

The shortest normal pregnancy (not considered preterm) lasted 247 days (35 weeks, 2 days), and the longest lasted 284 days (40 weeks, 4 days).

The researchers also learned that conceptions that took longer for the embryo to implant in the uterus also took longer between implantation and delivery.

Progesterone, a hormone that the body produces in high amounts during pregnancy, was also measured in the women.

Women who had a rapid increase in progesterone at the start of their pregnancies had pregnancies lasting up to 12 days longer until birth than women whose progesterone levels increased more slowly.

Older women were more likely than younger women to have longer pregnancies. In addition, women were more likely to have a longer pregnancy if their past pregnancies had lasted longer or if the women had heavier birth weights when they were born.

The length of the pregnancy did not appear related to a woman's (adult) weight, alcohol intake, number of previous children or the sex of her child.

This study was limited by the small number of women studied, so larger studies would provide more information about how the length of a normal pregnancy might vary.

"Variability in the length of human gestation limits the ability to predict delivery dates," the researchers wrote. "Nonetheless, it is common clinical practice to assign pregnant women a due date based on 280 days from the last menstrual period."

The authors suggested that an approach based more on the evidence would assign women a range of due dates or to describe the due date as a midway point between the range of possibilities.

Doctors might also take into account the length of a woman's previous pregnancies in approximating the length of a current pregnancy.

This study's findings confirm what many OB/GYNs already understand about the way "due dates" work, according to Andre Hall, MD, an OB/GYN at Birth and Women's Care in Fayetteville, North Carolina, and a dailyRx expert.

"An EDC (estimated date of confinement), more commonly known to the general population as the pregnancy "due date," is the date that is given to women as the date their baby is expected to arrive," Dr. Hall said.

"This is often misleading as very few babies are born on their actual due date," he said. "As a result, women schedule baby showers, family arrivals, etc. all around this date."

Dr. Hall added that a due date that comes and goes can then needlessly cause distress in a pregnant woman.

"A pregnancy due 'range' probably more accurately reflects reality as babies routinely are born between 37 and 41 weeks," he said. "When thinking in these terms, much of the angst surrounding the due date can be alleviated."

This study was published August 6 in the journal Human Reproduction. The research was funded by the National Institute of Environmental Health Sciences at the National Institutes of Health. The authors declared no conflicts of interest.

Review Date: 
August 6, 2013
Last Updated:
August 8, 2013