A full-term pregnancy is described as lasting for 40 weeks or 280 days, but women can have varying pregnancy lengths. One study found that the length of pregnancy can vary between women by an average of five weeks or 37 days.
Genetics plays an obvious role in determining the appearance of a baby, but do genetics play a role in determining a woman’s pregnancy length or duration?
Preterm delivery occurs when a baby is born before 37 weeks gestation. Late-term delivery occurs at 41-42 weeks gestation. Post-term delivery is another name for past due delivery at greater than 42 weeks gestation.
Several reasons for shorter or longer pregnancies have been described.
- Inaccuracies in estimating gestational age of the baby
Due dates are generally considered estimates. They are determined by measuring the baby’s size using ultrasound during early pregnancy. Another way to calculate a woman’s due date is based off of the first day of her last menstrual period. Basing the due date on a woman’s menstrual period can be incorrect if a woman doesn’t remember the right date of her last menstrual period.
- Genetic differences and natural variations in growth and development of the baby
Genetic traits of the mother, father, and/or baby may be associated with differences in pregnancy length between women. Environmental influences can also affect pregnancy duration. For example, poor prenatal care is linked to preterm birth.
Several studies suggest a correlation between the genetics of the mother, father, and/or baby and pregnancy length. Some recent studies evaluating the connection between genetics and pregnancy length include the following:
- A recent study published in the New England Journal of Medicine analyzed over 43,000 women with European ancestry who were part of a study conducted by 23andMe. The women reported the length of their first pregnancy. Age of the mother was associated with the duration of pregnancy. Genetic analysis of the 43,000 women and replication of genetic analysis in a sample of women from Nordic countries confirmed the presence of several locations and genes within DNA that were associated with length of pregnancy and preterm birth.
- A cohort study in 130 pregnancies reported that older maternal age, prior pregnancies with long durations, and heavier weight at birth were associated with length of pregnancy.
- A Norwegian study linked birth data from parents to birth records of their first born child to determine how genetic traits from parents can influence their offspring. The study found that length of pregnancy in first born children was associated with the length of pregnancy that defined their parents’ births. Shorter length of pregnancy was associated with a higher birth weight in the father.
- A study based on the Swedish Twin Registry analyzed the pregnancies of identical and fraternal pairs of female twins who gave birth between 1973 and 1993. Birth weight, gestational length, and growth were more similar between identical vs fraternal twin mothers, indicating that these traits have a genetic or heritable component. This study estimated that 25% to 40% of variations in traits such as gestational length are dependent on genetics.
- A Swedish study of more than 475,000 births found that 53% of variation in post term births was not related to genetic factors, while the other 47% of variation was accounted for by 26% fetal genetics and 21% maternal genetics.
There is still a need for further study to identify the exact effect of genetics on pregnancy length. Currently available studies are small and have many limitations that may prevent widespread applicability of results. Remember that your pregnancy length may be different from the length of other women’s pregnancies because of genetics and environmental factors. Whatever the cause, preterm and post term deliveries can occur. The American College of Obstetricians and Gynecologists provides information for women on preterm and post term births.