How long is human pregnancy? Your doctor will probably give you a response that goes something like this: “Term is considered thirty-eight to forty-two weeks after the first day of your last period.” But only four percent of mothers deliver on their due date (280 days from the first day of their last menstrual period, or LMP), even when using ultrasound for dating purposes.
You might wonder why doctors calculate gestation from the LMP, since ovulation and fertilization typically takes place approximately two weeks later. Researchers from the National Institute of Environmental Health Sciences (NIEHP) in Durham, NC, wanted to explore this question. After all, ovulation may coincide with fertilization and mark the actual start of pregnancy. But early events such as ovulation, fertilization and implantation make determination difficult.
Hormones Detected in Urine Reveal When Ovulation Occurs
The North Carolina study took a second look at data that researchers collected thirty years ago. Between 1982 and 1985, a group of mostly white, well-educated, non-smoking, normal weight women agreed to participate, going off their usual methods of birth control in order to become pregnant. Of these, 130 conceived. The women had an average age of 29 and more than half had other children. Subjects provided daily urine samples first thing in the morning until eight weeks after they became pregnant.
Researchers analyzed the samples for levels of three hormones; changes in these indicate when ovulation and implantation of the embryo took place. In early pregnancy, the fertilized egg releases human chorionic gonadotropin, or HCG; its detection makes your urine pregnancy test positive. HCG allows the area on the ovary which released the egg to produce progesterone to prevent pregnancy loss until the placenta takes over.
In 2010, scientists had enough data to determine the rates in the rise of progesterone in 60 of the births. Unlike our animal cousins the primates, it turns out that these hormone levels may rise quickly, slowly or not at all in the six days following implantation. Mothers whose progesterone levels climbed more slowly in the first weeks of gestation had longer pregnancies. Embryos that took longer to implant also took longer from conception to delivery. It seems that individual rates of fetal development may account for some of these differences.
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Normal Length of Pregnancy May Vary by 37 Days
The study found that the time from ovulation to birth spanned from 247 to 284 days or from 35 weeks and 3 days to 40 weeks and 6 days in healthy pregnancies. If a doctor approximates the LMP dating method by adding 14 days, he can estimate that you could deliver between 37 weeks plus 3 days and 42 weeks plus 6 days and still fall within the normal range of pregnancy length, which researchers found varies by as much as 37 days.
Fortunately, 120 of the mothers were available to help scientists revisit the data, determine the actual length of pregnancy, and look for any associated factors. In follow-up interviews, the women contributed information on the average length of other pregnancies that occurred before and after the original study and their own birth weight. It turns out that the average length of your other pregnancies (number of days all added together divided by the number of babies born) is a close predictor of how long your current pregnancy will last.
Decoded Pregnancy asked researchers Anne Marie Jukic and Allen Wilcox of NIEHS if pregnancy lengths could run in the family; i.e., will you carry your little one as long as your mother and sisters carried their babies? “This is a great question,” they said, “We had no data on the pregnancy lengths of immediate family members and could not address this in our study, but it would be interesting to study in the future.“
The scientists would also like additional research to include more diverse populations because they could not determine whether race or ethnicity independently affects gestational length. One earlier study showed that black women delivered an average of 8.5 days earlier than white women of similar socioeconomic status.Decoded Pregnancy
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