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 period is the first step in establishing your estimated due date (EDD). Our pregnancy due date calculator uses two methods: Nigel’s rule and Wood’s method, also known as Nichols’ rule. Why are we sharing both results? Most doctors and midwives follow the standard calculation of 40 weeks, Nigel’s rule, to determine a woman’s due date. We want you both to be on the same page. However, Nagiel’s rule makes each woman’s cycle typically 28 days long, and does not take into account whether or not this pregnancy is her fifth child.1 This makes the result false.
We’ve also included Nichols’ rule in the pregnancy due date calculator to calculate the average length of a woman’s due date and whether this pregnancy is her first child. By sharing both results, you’ll know when your expected due date is (for your health care policy) and from that date you can more realistically expect your baby to be born. (Nicholas’ rule)
Ultrasound confirmation of due date in early pregnancy
Once your last menstrual period establishes your EDD, it’s important to confirm it with an early prenatal ultrasound. ACOG states, “Ultrasound measurements of the fetus or fetus in the first trimester (up to 13 6/7 weeks of gestation) to establish or confirm gestational age.” It’s the best way.” ACOG has specific guidelines for confirming EDD based on ultrasound dating and menstrual dating.5
Can your baby’s due date change?
Once the 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 ACOG guidelines to accurately date your pregnancy, which includes dating your period using calculators, and then confirming with an ultrasound. Period dating can be unreliable, especially if you have irregular cycles, because it misses your conception date or ovulation time.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 the egg. If you have intercourse on the day of ovulation, you can get pregnant on that day. However, if you are having intercourse for five days after ovulation, you will not impregnate the UNGG and the sperm can fertilize it. On the same day you conceived your child.
How likely are you to go into labor on your baby’s due date?
Your chances of delivering your baby on your expected due date are very low. This is why it is estimated that there is an estimate. Only 5% of women give birth on their due date.3 This does not account for mothers carrying multiples who usually 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 moms deliver eight days past their due date.4
