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Nine Month Pregnancy? Maybe Not a New Study Says

Katlyn Joy |11, August 2013


It's something we all take for granted. There are nine months of pregnancy. Forty weeks. It's OK to go a couple early or late but that's the range. It's established medical fact and something we never even question.

However, a new study sheds a new light on the facts. Research published in this week's leading European reproductive medicine journal, Human Reproduction, found that human pregnancy may vary up to 5 weeks around that 40 week mark.

The study came out of the Epidemiology Branch at the National Institute of Environmental Health Sciences in Durham, USA which is part of the National Institutes for Health. Researchers looked at 125 singleton pregnancies which were naturally conceived by healthy women with no fertility issues. Daily urine samples were obtained from the women until the end of the eighth week of pregnancy. The women also kept journals on a daily basis.

The reason for the urine samples was so that three hormones could be tested for: hCG or human chorionic gonadotropin, estrone-3-glucoronide and pregnanediol-3-glucoronide. These hormones are all associated with pregnancy.

Researchers could determine by studying the hormone levels in the subjects the exact time of ovulation and implantation of the embryo into the uterus. Ovulation is pinpointed by looking at when there is a drop in the levels of progesterone and estrogen. Implantation is determined by looking at the first day when hCG levels rise.

The women studied were pregnant during the years 1982 to 1985, and researchers then reconnected with the women again in 2010. The women were asked when they went into labor and if their labors were induced or a c-section was required. From this data the researchers determined an average of 268 days from ovulation to birth. This is 38 weeks plus two days.

What surprised researchers was the great variety in gestational periods for the women. After taking out six premature labor and births from the study, researchers observed differences as much as 37 days in the lengths of pregnancy.

Doctors give women due dates of 280 days from the date of their last menstrual period. Of pregnant women, only 4 percent deliver on the exact due date given. Another 70 percent deliver within 10 days of that due date.

Dr. Virginia Beckett, a spokesperson for the Royal College of Obstetricians and Gynaecologists said not much is actually known about precise gestational periods. "This is a very interesting piece of work and knowing when is the right time to deliver is a huge issue," said Beckett, who herself did not participate in the study in any way.

Dr. Anne Marie Jukic, a postdoctoral fellow in the Epidemiology Branch at the National Institute of Environmental Health Sciences who was one of the researchers in the study said, "We were a bit surprised by this finding. 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. It's fascinating."

Certain patterns also emerged in the study, such as when women took longer for their embryos to implant in their wombs, their pregnancies were also longer. Progesterone levels were also closely tied to the length of labor with women who had early rises in progesterone levels giving birth 12 days later than women with late rises.

Older women were observed to have longer gestations, with each additional year of mother's age resulting in one additional day of pregnancy compared to younger moms.

Women who weighed more at birth themselves tended to have longer pregnancies with each 100 gram increase in weight corresponding to an additional day of pregnancy.

Researcher Dr. Jukic found it interesting that early event in pregnancy may have a great deal to do with labor and birth outcomes. "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."

Beckett found this to be one way to take some pressure off expectant mothers and give them one less thing to worry about. She said the study shows a due date is not an exact science and women should not worry when theirs comes and goes without labor starting. "It would be better to say, 'You will be delivered by this time to take the pressure off.'"

Katlyn Joy is a mother to 7 children, and a freelance writer. She earned her Master of Arts in Creative Writing and Poetry, and a Bachelor of Arts in English and was previously an adviser to new mothers on breastfeeding through a maternity home program. She currently resides in Colorado with her family.

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