Your Due Date Is a Guesstimate

Due date

A good friend of mine is an obstetrician. She told me a story of something that happened back when she was a resident in obstetrics.

A woman came into the emergency room of the large city hospital where Jeannie worked and said she was having a baby. The emergency room was slammed that day with patients coming in from all over with all kinds of problems. The triage nurse didn’t have time to ask too many questions and the woman looked comfortable, so the nurse sent her right up to the OBGYN floor, which was also busy.

My friend was the resident on duty. As she walked into the room, she saw a healthy and very pregnant woman who was looking comfortable and happy. Most women who are about to deliver a baby have had a few hard contractions already or at the least look uncomfortable.

The first question Jeannie asked the woman was when her last contraction occurred. The woman said that she hadn’t had one.

Thinking that she didn’t understand the question, Jeannie asked, “How many contractions did you have before you reached the hospital and how far apart have they been.” The woman replied that she wasn’t having any contractions, but that she was there now to have her baby.

“Has your water broken?” “No.”

Jeannie asked who the woman’s obstetrician was and if she had been scheduled for a cesarean section. The woman gave the name of an obstetrician on staff and said that she hadn’t been planning on a cesarean section.

A quick exam found that the woman was not in labor. Jeannie asked, “So why did you come in now? Are you in any pain? Is there a problem?”

The woman smiled widely and said, “No. But today is my due date and I came here to have my baby.”

While this woman was a bit more naïve than most pregnant women, many women still forget that their due date is an estimate. It is a very good estimate, but it is basically a guideline, not a promise you are going to deliver on that day. In fact, according to some sources, only between 4 and 5% of mothers actually deliver on their estimated due date.

A pregnancy usually comes to a natural delivery at about 280 days–40 weeks–after the first day of your last menstrual period. That method of estimating a due date has been used since the early 1800s and works reasonably well. You can check out a Pregistry blog post about due date calculators here.

A better method of determining an estimated due date is to use ultrasound images in the first trimester to measure the skull of the fetus. If the due date calculated with the calendar method differs from that found with the ultrasound method, the ultrasound due date is the one used.

Knowing the estimated gestational age of a fetus is important. Giving birth too early—generally considered to be before 37 weeks—is not good for the baby. There are also possible problems with going more than two weeks beyond the estimated due date. The traditional rule of thumb is that a baby is considered full term if he or she is born up to three weeks early or up to two weeks late.  The American College of Obstetricians and Gynecologist defines a full-term birth a bit more narrowly, as being one that occurs between 39 weeks and 40 weeks and 6 days, which is almost 41 weeks.

So take that due date with a grain of salt. Have your “go bag” ready by week 37, just in case, and don’t be so hung up on a particular date on the calendar.

Valerie DeBenedette
Valerie DeBenedette is an experienced health and medical writer who lives about an hour north of New York City with a dog that is smaller than her cat. Her work has appeared in magazines, newspapers, newsletters, and on websites. She is a member of the National Association of Science Writers.

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