Your due date is not 100 percent accurate. Find out why you may meet the new baby a few days before or after your scheduled date.

By Dr. Laura Riley
Updated: February 12, 2019
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From the time you learn you're pregnant, the countdown to your due date begins. But the seemingly simple question "When are you due?" can really be answered by only one person: Your baby.

For earlier generations of women, the concept of a due date was "around Thanksgiving" or "late fall." As birth moved from home to hospital, women were given the approximate date when they should expect to be confined to a hospital bed, called the "estimated date of confinement." Eventually, this term evolved into "expected date of delivery," now called "due date."

Unfortunately, a specified due date has made women (and their family and friends) place too much emphasis on a precise day – to the point that they plan their life around it. Your baby doesn't have a calendar, however, so it is no surprise that less than 5 percent of babies actually arrive on the date they are due. Most women deliver healthy babies anywhere between 37 and 42 weeks after getting pregnant. Here’s what you need to know about the reliability of due dates.

How Doctors Calculate Due Dates

Many health-care providers use a sonogram to pinpoint your due date, but don't be swayed by technology: A date based on an ultrasound can be off by a week or more depending on the skill of the technician, the timing of the sonogram, and the size of the baby.

Until 13 weeks of gestation, most babies grow at the same rate, but as pregnancy progresses, fetal size corresponds less and less to the amount of time that the baby is in the womb. So while many health-care providers keep giving ultrasounds to reassess a woman's due date throughout her pregnancy, the date is actually becoming less accurate as time goes by. In fact, there's really no need for a sonogram to determine a due date unless you don't know the date of your last period.

If you do know the date of your last period, try the following calculation, called Naegele's Rule. Babies have a gestational period of about 280 days, so count back 3 months from the first day of your last period and add 7 more days. Your approximate due date is that day within the next year. Take note of the word "approximate:" Your baby will grow and mature on his own schedule. The only thing you'll know for sure is that you should give birth within two weeks before or after that day.

Guessing When Baby Will Come

So how do you know when your baby will arrive? You really don't until the Big Day. Until then, you can make an educated guess. For instance, if this is your first pregnancy, you're likely to deliver early if your mother did. Caucasian mothers tend to have the longest pregnancies, and so do women under 30. Girl babies are more likely to be born earlier than boys. Also, if you have longer menstrual cycles, you're more likely to deliver your baby after your due date – but you can never know for certain ahead of time.

Should I Schedule an Induction?

Many women (and their health-care providers) become so attached to their due date that when the baby doesn't come on that day, they schedule an induction. But because the due date is unreliable in the first place, inducing the baby may cause him to be born too soon. These "near-term" infants (as they are known) can have trouble breathing, staying warm, and breastfeeding, and they often need special hospital care after birth.

Induction isn't without risks for you, either: Research has shown that a first-time mother whose labor is induced is twice as likely to have a cesarean as one whose labor starts on its own. When you let your baby choose his own birthday, it means he's really ready to begin life outside the womb.

Remember: Your due date is an estimate of when you will give birth, not a guarantee. If you are "overdue," just count it as more time to establish a bond with your baby, to prepare mentally for motherhood, to discuss your partner's role during the birth, and to ready your home for your baby's arrival. Consider telling family and friends that you are due "sometime in May" rather than on a specific day in order to prevent a daily barrage of phone calls near the end of your pregnancy. Trust that your baby will know when the time is right for his big debut. It will be worth the wait for everyone.

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