How do we calculate your baby’s due date?
There are several ways to calculate a person’s due date.1 We use two methods in our due date calculator; 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. This makes the result often inaccurate.
We also include the Nichols’ Rule in our due date calculator because we want 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 health care provider (the Naegele’s Rule) and the date you might more realistically expect your baby to be born (Nichols’ Rule).
Can your baby’s due date change?
Absolutely. Your doctor or midwife will use one of the formulas mentioned above to calculate your due date. They will also measure your baby’s growth by measuring your fundal height during each prenatal appointment and/or measuring your baby via ultrasound between 8 and 12 weeks gestation. Your baby’s due date may change if your baby consistently measures bigger or smaller than expected for your gestation. This can happen if you have irregular cycles because it can be more challenging to determine your date of conception or if you ovulate earlier or later than expected.
Note: Conception does not always occur on the date of intercourse. Sperm can live inside your body for up to 5 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 5 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 8 days past their due date.4
