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In order to help pregnant women during their last trimester and before the birth of a newborn child, doctors must be aware of their patients' length of pregnancy and the amount of fetal growth. For the longest time, doctors have estimated due dates based on the previous research. Now, according to new research, the length of pregnancies between women can vary up to five weeks.

For this study, the researchers from the U.S. National Institute of Environmental Health Sciences recruited 125 pregnant women and calculated their pregnancy lengths. The researchers used the pregnant women's urine to test for three hormones that have been tied to the beginning of fertilization. The presence of these hormones helps the doctors calculate exact moments of ovulation and fertilization, giving them a better way to estimate due dates.

"We found that the average time from ovulation to birth was 268 days - 38 weeks and two days," researcher, Dr. Anne Marie Jukic said according to HealthDay. Jukic is a postdoctoral fellow in the epidemiology branch at the U.S. National Institute of Environmental Health Sciences. "However, even after we had excluded six pre-term births, we found that the length of the pregnancies varied by as much as 37 days."

This finding is surprising because according to previous research, doctors worked under the belief that women give birth 280 days after the beginning of their last menstrual cycle. Despite this, the statistics only reveal that four percent of women actually deliver at 280 days while 70 percent of women give birth within 10 days of that time frame. Doctors use this understanding to help women estimate their due dates and monitor the health of mother and child before birth. However, this new research suggest that the pregnancy lengths can be varied up to five weeks, meaning that women can have perfectly normal and healthy births weeks before their estimated due date.

"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," Jukic explained. "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."

The researchers reasoned that the five weeks difference that they calculated could be due to a natural variability tied to mothers; individual bodies. The researchers noted that it is still too early to create clinical recommendations for doctors when they assess their pregnant patients. Whether or not estimated due dates are helpful could be subjected to review. However, more research needs to be done in studying this variability.

The findings were published in the journal, Human Reproduction.