Save the Date

All about your due date


So, when's that baby due? How many months along are you? Why did your ultrasound pick a different date? When is the baby old enough for induction? Are some women really pregnant for 10 months? These aren't always easy questions to answer. Readers want clarification on nailing down the due date, which is especially important when scheduling inductions.

Estimated due dates (EDD), sometimes still referred to in olden-days vernacular as the estimated date of confinement (EDC) are usually calculated based on the first day of your last menstrual period (LMP) and projected for 40 weeks from that day. It's a guess, ladies, not a commitment. I've had patients show up at the hospital on their due date (not in labor or scheduled for induction) thinking they had an appointment to have their baby. "My doctor said the baby would be born this day." Duh.

It's tricky figuring out exactly when conception happens. Lots of women know the exact magic moment but lots more haven't a clue. The due date is a guesstimate because—news flash—not every woman has a clockwork menstrual cycle. Sure, many can set their calendar for exactly 28 days but most of us aren't that regular. Calculating when we're ovulating is trickier still and sperm can hang out in the cervix for days waiting for an egg to appear. That means figuring out the precise moment when the egg got bombarded by sperm is a crapshoot. Typically, women with regular periods, conceive about 11-21 days after the first day of the last period. Baby is born some 38-40 weeks after that. Only 5% of babies are actually born on their due date.

Just to complicate matters further, ultrasounds also play a part in picking the big day. By taking measurements of the fetus and comparing them to the LMP they come up with a date that's pretty close to the LMP EDD. First trimester ultrasounds are more accurate than later ones for estimating fetal age.

My mom swore she was pregnant for 10 months with each of her eight children. Well, yes and no. She was pregnant for 10 lunar months—each one based on 4 seven-day weeks adding up to 28 days. The average pregnancy lasts 266 days from conception or 280 days from LMP. So, if she was going on that calendar—she was, indeed pregnant for 10 months. Most of us tell time by a calendar month of 30-31 days. By that standard, the average pregnancy is 9 months.

Due dates give us the ability to plan for the end game (birth) but also help calculate placental age and fetal lung maturity. The placenta is our only disposable organ. Unlike the heart, brain, lungs, etc. which last a lifetime, we make a fresh placenta for each pregnancy. Once the baby's born, its job is done. Unfortunately, not all placentas work at maximum efficiency for the entire duration of the pregnancy. If the placenta decides to quit work before delivery—that's a big problem. That's why doctors/midwives are reluctant for women to go too far past their due date. Once you're beyond 40 weeks, the placenta wants to retire. You'll probably get scheduled for nonstress tests (fetal heart monitoring) to determine if it's still functioning, as it should.

While the placenta may be done working too soon, baby's lungs might not be working soon enough. Lungs continue developing right up to the very end—specifically, the coating of surfactant (a lipid/protein material) that allows lungs to inflate. Babies born too early (usually younger than 36 weeks) are at higher risk for developing respiratory distress syndrome (also called hyaline membrane disease).

A reader asked if it's OK to schedule an induction for 38 weeks. She didn't mention any medical problems so I'm assuming she's just sick and tired of being pregnant and wants to get the show on the road. I know how she feels. I think the reason pregnancy lasts nine months is because that last month is so uncomfortable that labor seems like a Good Idea— something you Want to go through. Setting a precise date is mighty convenient but not always a great plan unless medically indicated. If yours or the baby's body isn't ready for delivery—you're signing up for problems.

Most doctors/midwives won't schedule inductions before 38 weeks without a darn good reason and even then, if the due date isn't absolutely concrete, there's risk you'll be delivering too soon. Think it through. A cervix that isn't ready for labor won't dilate which ups the chances of cesarean section. Baby lungs that aren't ready to breathe up the chances of time spent in the neonatal intensive care unit.

The brain does a funny thing to women who go past their due date. It convinces us that we really might be pregnant forever. Forever. At 10 days overdue with my first, I was certain I'd be the first woman in Ripley's Believe It or Not to be pregnant for the rest of my life. I'm a smart woman. I know that's not possible and still, I was convinced. The baby didn't come on her due date therefore she probably wasn't coming at all. It had all been a hoax. I had simply swallowed a watermelon seed and would remain a bloated, enormous being forever. Yeah, OK, so I freaked out a little. Not to worry, you really will have your baby. No one has ever been pregnant for their entire life, though with eight, 10-month pregnancies my mom came pretty close. And there's that woman who had her 17th baby last month and is looking forward to her next pregnancy. She's definitely gunning for the pages of Ripley's.

Got a question for Jeanne? E-mail it to and it may be answered in a future blog post.

This Fit Pregnancy blog is intended for educational purposes only. It is not intended to replace medical advice from your physician. Before initiating any exercise program, diet or treatment provided by Fit Pregnancy, you should seek medical advice from your primary caregiver.