Only 5 percent of women give birth when they expect. Most women–80 percent–deliver babies between 37 to 42 weeks of pregnancy, and about 11 percent of women deliver prematurely. So where does that put you? How do you calculate your pregnancy due date, and when will you actually give birth?
Due Dates 101: How to calculate your due date
Accurately predicting your due date is a lot more complicated than counting nine months. The human gestation period takes around 38 to 40 weeks (which is actually longer than nine months!). At your first prenatal visit, your doctor will give you an official due date, based on a number of factors.
How to calculate due date based on your last period
The average pregnancy lasts about 280 days or 40 weeks from the first day of your last menstrual period. Typically, doctors will use this average gestation period to calculate your estimated pregnancy due date.
Human Gestation Formula
| [First Day of Last Menstrual Period] + 280 days = Due Date Based on Last Period |
For example, if your last menstrual period began on May 20, your estimated pregnancy due date would be February 24, 280 days later.
Naegele’s Rule Formula
Another way to calculate the delivery date is a formula called Naegele’s rule. Most pregnancy calculators found online are based on this rule. It’s simple: count back three months from the first day of your last period and add 7 days.
| [First Day of Last Menstrual Period] - 3 months) + 7 days = Due Date Based on Last Period |
For example, if your last menstrual period began on June 6, your estimated due date would be March 13.
The problem with calculating your pregnancy due date based on your last period is that both the human gestation formula and Naegele’s rule are based on a variety of averages. Pregnancy calculators are typically based on a 28-day cycle and assume that ovulation occurred on day 14. But many women have longer or shorter cycles and ovulation tends to occur later rather than earlier. Earlier ovulation means you’ll give birth earlier; later ovulation means you’ll give birth later than your expected due date.
How to calculate due date by conception
If you know your date of conception (aka when the sperm met the egg), you can count 38 to 40 weeks out to calculate baby’s due date. But here’s the big mystery: when exactly did fertilization occur? You may know the date of intercourse, but that’s not necessarily the date of conception. Sperm can live up to five days in the fallopian tubes before fertilization. During your six days of fertility, you may not have conceived unless you were ovulating. So unless you’d been using an ovulation tracker or were able to make a baby in the first go, knowing the date of conception isn’t that clear cut.
Any who, we’ll still give you the formula just in case you do actually know when you conceived.
Due Date by Conception Formula
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[Date of Fertilization or Conception] + 38 to 40 weeks = Due Date Based on Conception Date |
For example, if you conceived on October 15, your due date would be around July 8 to July 22.
How to calculate due date by ultrasound
Ah yes, modern technology: the ultrasound, we love you. Your doctor can help calculate your pregnancy due date via ultrasound, which is especially helpful if you have irregular periods. After your first prenatal visit, your healthcare provider will schedule an ultrasound to confirm the pregnancy.
During an ultrasound, your doctor will measure baby’s length–from crown to rump–to determine how many weeks along in your pregnancy you are and calculate your due date.
This gets a bit complicated, however. Babies tend to grow at various rates, so some may arrive 2 to 3 weeks before or after the predicted due date.
Your doctor will monitor baby’s growth during the first trimester based on the expected gestational age from your last menstrual period.
Ultrasound Formula
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(Technology needed) No simple formula here! Due date is based on the first day of your last menstrual period as well as baby’s size and growth rate during the first trimester. |
How to calculate IVF due date
If you conceived via in vitro fertilization, you’ll know exactly when the egg was fertilized. As a result, IVF due date calculation can be pretty precise. Due date is determined either by your egg retrieval day, your intrauterine insemination (IUI) day, or if you’re having a three or five day embryo transfer.
IVF Embryo Transfer Formula
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[Date of IVF Embryo Transfer] + (266 days - [Day Three Embryo or Day Five Embryo]) = Due Date Based on IVF Embryo Transfer |
For example, if you had a three-day embryo transfer on April 3, your due date would be December 22.
IUI Formula
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[Date of IUI] + 38 weeks = Due Date Based on IUI Date |
For example, if your IUI day was Jan 11, your due date is October 4.
Why calculate a due date at all?
Calculating your due date is important for a number of reasons. Knowing your due date can help prevent the need for labor induction if post-term (more than two weeks late) or attempt to stop preterm labor (before 37 weeks). Numerous tests and procedures are also based on baby’s gestational age.
What if I have multiple due dates?
It’s not uncommon to be told (or to calculate) multiple due dates. Always talk to your healthcare provider about your due date and make sure to understand how each due date was calculated.
Also remember that although your due date might be in Mother Nature’s hands, you do have control over how you care for your body and baby during pregnancy.
Reminder: take your prenatal vitamin!
