The Nine – Month Rule Of Thumb For Pregnancy Can Differ Up To 37 Days, Finds Study

Many of us are under the impression that a woman delivers within nine months, but the latest findings show that the actual period of gestation can differ by as much as 37 days.


Obstetricians normally give a birth date that is 280 days after a woman’s last menstrual cycle. But, it has been found that only 4 % of women deliver on the given date, while 70 % of them deliver within nine to ten days of the estimated date.

“We are still not clear on how long a woman would take to deliver. However, this is really an interesting research and knowing when the exact time to deliver is a big issue”, admitted Dr. Virginia Beckett, gynecologist and spokesperson for the Royal College of Obstetricians.

However, some scientists from the National Institute of Environmental Health Sciences (NIEHS) were able to find out the exact moment when women became pregnant by testing their daily urine samples. This test enabled them to pinpoint the role that natural variation plays in the length of pregnancy.

The Research:

For the study, researchers followed 125 single- birth pregnant women, who conceived naturally. The subjects were healthy with no fertility issues and less likely to smoke or be obese. These women were asked to give their daily urine samples until the eighth week of pregnancy.

In their urine samples, researchers looked for three hormones associated with pregnancy. This included -hCG (human chorionic gonadotropin), pregnanediol-3-glucoronide, and estrone-3-glucoronide. By using this info, the researchers came across the exact time when a woman ovulated and the time the fertilized embryo embedded itself in the womb. The day of ovulation was, when there was a fall in the ratio between progesterone and estrogen hormones, while implantation was determined as the first day of the rise in hCG hormone.

The normal period taken from ovulation to birth was about 268 days, which equals 38 weeks and 2 days. However, when the researchers discounted six pre-term births, they found that the time taken for pregnancies differed as much as 37 days.

Factors that could affect the Length of Pregnancy:

Older and obese women had longer pregnancies. In addition, the researchers found that mothers who had embryos took more time to implant and had longer gestation periods. On the other hand, pregnancies that had delayed increase in the levels of progesterone were shorter as compared to those who had an early rise.

“The time taken for a child’s growth varies greatly among healthy pregnancies, even when ovulation is exactly measured. However, this variability is greater than proposed by the clinical assignment of a single ‘due date'”, added the researchers.

This finding shows that science is still evolving, researchers said. They even added that giving women a specific “due date” for their pregnancy often causes anxiety when that day comes or passes. Thus, it would be better to say that they would be delivering by this time. This could help ease off the pressure.


Researchers were intrigued by the observation that occurs very early in pregnancy, but give important information about the due date, which occur months later. This simply suggests that events in early pregnancy may give a new pathway for examining birth outcomes.