How to Determine a Baby's Due Date

Determining a baby's due date is usually just a matter of simple calculation. An irregular menstrual cycle or other factors might make it more difficult, though. Prenatal caregivers need an accurate due date for testing, but babies come when they want to, with only 5% being born on their due dates.

Instructions

1. Count 40 weeks past the first day of your previous menstrual cycle. That's the simplest method, and the one doctors use. Most online pregnancy due-date calculators use this method, too.

2. Count 38 weeks from the date you ovulated if your menstrual cycles are irregular. Ovulation occurs about 14 days before a period is due, so women with a longer cycle don't always ovulate two weeks after their period. Some women feel a physical sensation from ovulation. You can also use an ovulation predictor if you're trying to conceive.

3. Ask for an ultrasound to date the pregnancy by visiting your obstetrician. An ultrasonic wand is placed on your abdomen. The operator clicks on either side of the growing amniotic sac to measure and can date a pregnancy very early, then can determine baby's due date.

4. Calculate 38 weeks and three days past the date of sexual intercourse. This method works well when you're not having sex very often. The 38 weeks is the gestation after conception and the three days is a cushion. Sperm can live in the body for up to 72 hours, and it takes several hours to swim from the cervix to the egg.

Tips & Warnings

• Keep in mind that while U.S. pregnancies average 39 weeks, that figure includes inductions and elective Cesareans. Babies that aren't induced or prematurely removed from the womb tend to gestate for an average of 41 1/2 weeks.