Ask any mom whose pregnancy went into extra innings: Right around your due date, the phone calls, emails and texts start coming: “Is anything happening?” “What does the doctor say?” “Wellll????” Playing the waiting game during the last few weeks of pregnancy is hard, but it becomes especially difficult when 40 weeks turn into 41 . . . or 42. But the first thing to know is this: Your due date is just an estimate. In fact, only 5 percent of babies are born on theirs.
“No woman should feel nervous or anxious if she’s still pregnant after her due date,” says Alex C. Vidaeff, M.D., M.P.H., a maternal-fetal medicine researcher and practitioner at the University of Texas Medical School at Houston. “Due dates can be off by a week in either direction.” Sometimes, even two weeks! Despite that fact, many women get hung up on those dates and feel distress when they come and go, Vidaeff adds.
In the first trimester, your due date is calculated by adding 40 weeks (280 days) to the first day of your last menstrual period, then synched with ultrasound images that date the age of the embryo or fetus. But the result is just an educated “guesstimate.” “You might really be 39 weeks when you think you’re at 40,” says Vidaeff, adding that pregnancy length is, in many cases, genetically determined.
Going into overtime The strict medical definition of “post-term” (aka “post-date”) is a pregnancy that goes beyond 42 weeks. However, recent research has shown that delivery after 40 weeks may come with certain risks, and OB-GYNs have responded in kind. “We now change our clinical practices at 40 weeks to prevent potential complications in both mom and baby,” says Carri R. Warshak, M.D., an assistant professor in the department of maternal-fetal medicine at the University of Cincinnati.
At the 40-week mark, your doctor or midwife will become more vigilant about monitoring the baby. Post-term infants are at higher risk for problems with glucose metabolism, which can lead to excessive birth weight, and for inhaling meconium (fecal waste); this can cause breathing problems or infection at birth. Amniotic fluid levels can drop, and after 42 weeks, the placenta may not work as well as it did earlier.
Your caregiver will regularly check your amniotic fluid level, the baby’s heart rate and his estimated weight, which increases significantly from 40 to 42 weeks. “If the fetus weighs more than 8 pounds, 13 ounces, you may have difficulty pushing during labor, and using a forceps or vacuum may be necessary for delivery,” says Warshak.
When baby needs a nudge After 39 or 40 weeks, it’s sometimes best to deliver sooner rather than later: Studies have shown that newborn admissions to the neonatal intensive care unit (NICU) increase slightly when a pregnancy lasts 40 to 42 weeks. And stillbirth, though still rare, becomes a concern: At 40 weeks, the risk is 2 to 3 per 1,000 babies; at 42 weeks, it’s 4 to 7 per 1,000. If labor hasn’t started spontaneously, you’ll probably be induced at 41 weeks—at the latest. “If your cervix has started to dilate, odds are in your favor for a successful induction,” Warshak says. Translation: You won’t end up having a Cesarean section.