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Women Pregnancy - The 9 months myth

Updated on August 12, 2013

Pregnant woman



Pregnancy is the fertilization and development of one or more offspring, known as an embryo or fetus, in a woman's uterus.

In a pregnancy, there can be multiple gestations, as in the case of twins or triplets. Childbirth usually occurs about 38 weeks after conception; in women who have a menstrual cycle length of four weeks, this is approximately 40 weeks from the start of the last normal menstrual period (LNMP).

Human pregnancy is the most studied of all mammalian pregnancies. Conception can be achieved through sexual intercourse or assisted reproductive technology.

Human Reproduction studies - interesting conclusion

Contrary to most people's beliefs, the human pregnancy is highly variable, and these variations may be unexpectedly high - up to 5 weeks, according to a recent study.

In the study published in Human Reproduction, researchers presented a surprising conclusion for many people: the idea that the human gestation period would be 9 months with a variation of only a few days, as commonly believed, is erroneous. In fact, pregnancy time may vary, naturally, with up to 5 weeks.

Typically, the date of birth is based on the date of your last period started, adding 280 days from that date.

However, only 4% of women give birth after 280 days from the date of their last menstrual period and only 70% in the next 10 days, even if the date of birth is estimated more precisely using ecography.

Human Embryo


Researchers found the precise birth time

Now, for the first time, a team of American experts from the National Institute of Environmental Health Sciences could establish the precise moment when a woman ovulates and when the fertilized egg implants itself into the uterus if a pregnancy occurred naturally, and they kept under observation the pregnancy until birth. In this way, they could determine precise times of 125 pregnancies.

The study conclusions

Here are some of their findings:

  • the average length of time between ovulation and birth was 268 days (38 weeks and 2 days);
  • however, after excluding the 6 pregnancies which were completed with premature births, they found that the length of pregnancy periods vary by up to 37 days;
  • older women tend to give birth later, each additional year corresponding, on average, with one day in addition until birth;
  • women who had more weight than usual when they were born tend to have longer pregnancies;
  • a woman has almost the same pregnancy period every time she is pregnant.

The pregnancy variations

Until now, although it was known that pregnancy can vary in duration, variations were attributed mostly, to the pregnancy estimation errors. But U.S. researchers say they have measured very precisely the pregnancy period, from ovulation to birth, yet they found large variations, over five weeks - a result so surprising that they have declared themselves fascinated by this discovery.

The possibility of the pregnancy natural variations have been less studied, because until now, it was impossible to make a difference between natural variability and miscalculations, since it could not be absolutely accurate to measure the embryo's age.

Hormone analysis was the key

Researchers were able to monitor the pregnancy outcomes accurately by analyzing three hormones that could be found in urine samples from pregnant women who had previously participated in another study, the North Carolina Early Pregnancy Study, which took place between 1982-1985.

Using data from this study on hormone levels in the urine, the researchers were able to identify when each woman got pregnant.

The three hormones analyzed - hCG (human chorionic gonadotropin), estrone-3-glucoronida and pregnanediol-3-glucoronida - their specific variations in urine concentration may indicate when the woman's body enters the state of pregnancy.

In 2010, researchers contacted the women who participated in the 1980 study and they have asked for details about how was the birth. They excluded the birth cases which were caused and also the scheduled C-sections, as well as the cases in which women were exposed to diethylstilbestrol - a chemical compound that affects the endocrine system and shortens the duration of pregnancy. In this way, they calculated the natural length of 125 pregnancies.

Baby kicking


In conclusion

Although researchers say it is too early to make clinical recommendations based on these results, further studies are needed in this area, however, they consider very important that the human pregnancy have a much greater variability than previously thought, and doctors would need to have this in regard when deciding whether to intervene or not in the pregnancy process.


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