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Countdown to baby: How your pregnancy is dated

How your doctor estimates your due date.

As soon as your friends and family know you're pregnant, the first question is likely to be, "When is the baby due?"

The truth is, very few babies are born on their due dates. But the date is still important. It can help you plan for the birth. It's also a guide for checking how your pregnancy is progressing.

Do the math

To estimate due dates, doctors assume the baby will be born about 38 weeks after conception. Most women don't know exactly when that happened. So due dates are based instead on the first day of your last period. That makes the total time until birth about 40 weeks.

Some simple math can help you find your due date. Just add 40 weeks to the first day of your last period. For example, if that date was Jan. 1, your due date would be 40 weeks (280 days) later on Oct. 7.

You can also use this due date calculator.

The start of your last period also determines your baby's gestational age. Your doctor will use that age to check on how your baby is growing and developing.

When memory fails

Finding your due date can be difficult if your periods are irregular. Or you might not remember the first day of your last period. But there are other ways to find out your due date. They include routine pelvic exams and ultrasound tests. These tests measure the size of the uterus. That measurement is used to figure out how far along you are.

Babies can be measured this way very early. But the best time is between the 8th and 18th weeks of pregnancy, says the American Pregnancy Association.

Using tests to find your due date is harder later in pregnancy. That's one more reason to start prenatal care early.

The long and short of it

Your baby can be born as much as three weeks before or two weeks after your due date and still be within the normal range, says the American College of Obstetricians and Gynecologists.

If your labor starts before 37 weeks, it's considered preterm. About 1 in 10 babies is born this early, says the American College of Physicians. For about half of them the cause is unknown.

For the rest, the trigger may be an infection or a health problem, such as diabetes. Preterm birth is also more common in women who smoke. Women who are having more than one baby are also more likely to give birth early.

It's also common to go past your due date. In fact, women having their first babies often give birth later than expected.

If you go a week or more past your due date, your doctor will keep a close eye on your baby. He or she may:

  • Check your baby's heartbeat.
  • Order an ultrasound exam to look at the amniotic fluid around your baby.
  • Check your cervix to see if it's becoming dilated and thinned.

Depending on what these tests show, your doctor may decide to induce your labor.

You may also need to be induced if you have a chronic condition such as high blood pressure or diabetes, reports the Office on Women's Health.

Getting it started

There are several ways to help start labor. One is to rupture the membrane that holds the amniotic fluid. This is also called "breaking the water." It's not painful and usually brings on labor.

Another way is to use a medicine called oxytocin. This is a hormone that helps start contractions. Contractions usually begin within a few hours after the medicine is taken.

Your baby will be monitored carefully throughout this process.

Running late

If baby is running late and you are past your due date, don't worry. Most babies born after their due dates are healthy.

reviewed 12/24/2019

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