How Do We Calculate Your Baby’s Due Date?

There are several ways to calculate a person’s due date.¹ Traditionally, determining the first day of your last menstrual period is the first step in establishing your estimated due date (EDD). Our due date calculator uses two methods: Naegele’s Rule and the Woods Method, also known as Nichols’ Rule. Why are we sharing both results? Most doctors and midwives follow the standard 40-week calculation, Naegele’s Rule, to determine a woman’s due date. We want you both to be on the same page. However, Naegele’s Rule generalizes every woman’s cycle to be 28 days long, and it does not consider if this pregnancy is her first child (that she’s given birth to) or if this is her fifth child.¹ This often makes the result inaccurate.

We also include the Nichols’ Rule in our due date calculator to account for the woman’s average length of cycles and whether this pregnancy is the first child. By sharing both results, you will know when your estimated due date is for your healthcare provider (the Naegele’s Rule) and the date you might more realistically expect your baby to be born (Nichols’ Rule).

Ultrasound Confirmation of Due Date in Early Pregnancy

Once your last menstrual period establishes your EDD, it is essential to confirm it with an early prenatal ultrasound. ACOG states, “Ultrasound measurement of the embryo or fetus in the first trimester (up to and including 13 6/7 weeks of gestation) is the most accurate method to establish or confirm gestational age.” ACOG has precise guidelines for confirming the EDD based on ultrasound dating and menstrual dating.⁵

Can Your Baby’s Due Date Change?

Once ultrasound confirms your EDD, it is unlikely that your due date will change. Ultrasound dating is more accurate than menstrual dating, especially when done in the first trimester. Your doctor will follow the ACOG guidelines to accurately date your pregnancy, which includes menstrual dating using the calculators, and then confirmation with ultrasound. Menstrual dating can be unreliable, especially if you have irregular cycles, because it can be more challenging to determine your date of conception or timing of ovulation.⁵

Note: Conception does not always occur on the date of intercourse. Sperm can live inside your body for up to five days.² It all depends on when you ovulate and release an egg. If you have intercourse on the day of ovulation, you may conceive that day. However, if you have intercourse up to five days before ovulation, you will not conceive until your body releases an egg and the sperm can fertilize it. That is the day you conceived a baby.

How Likely Will You Go Into Labor on Your Baby’s Due Date?

The chances of you delivering your baby on your estimated due date are slim. This is why it’s called an estimated due date because it is precisely that, an estimate. Only 5% of women give birth on their due date.³ This doesn’t account for mothers carrying multiples who typically deliver two weeks earlier. For mothers carrying singletons, you can expect to give birth within two weeks before or after your due date. On average, first-time mothers deliver eight days past their due date.⁴