As soon as you announce you’re pregnant, people want to know one thing: When is your baby due? You can wait for your doctor or midwife to calculate your due date for you, but if you know the first day of your last menstrual period, you can estimate it yourself.
Here’s how to DIY your due date in three simple steps:
1. Take the first day of your last period (let’s say it was August 1st)
2. Go back three months (that would be May 1st) and
3. Add seven days – That means your baby is due on May 8th.
What if you don’t know when you had your period?
That makes dialing in a specific date a bit trickier and you may need an ultrasound to figure it out. To get the most accurate due date via an ultrasound, you must have it performed within your first trimester – the earlier the ultrasound, the more accurate the date. If you have an ultrasound in your second or third trimester and your doctor or midwife uses that information to nail down when your baby will be born, don’t count on that due date being terribly accurate.
In fact, the American College of Obstetricians and Gynecologists just released new guidelines this week to help providers estimate more accurate due dates. ACOG says:
Overall, a high-quality ultrasound in a woman’s first trimester is the most accurate method of establishing or confirming the gestational age of the fetus. If an ultrasound performed less than 14 weeks into the pregnancy suggests a due date that differs by more than seven days from the estimated due date generated by a woman’s last period, the woman’s providers should change her due date to reflect her ultrasound. Before the nine-week mark, a discrepancy of more than five days is reason enough to change her due date.
Does that mean you must have an ultrasound to establish an accurate due date?
No, if you know when your period started, that should be enough information to estimate a solid due date. But here’s the thing – your due date is just an estimate, not a contract. Your baby will most likely be born near that date but it’s important to remember that only five percent of women deliver right on their due date. The rest deliver a little early or a little late and that’s entirely normal. Two weeks before a due date to two weeks after is considered a safe range for spontaneous labor to start.