Due Date Calculator

So you’ve found out you’re pregnant—congrats! For many moms-to-be, the next thing on their minds is “when is my due date?” But accurately calculating your pregnancy due date isn’t quite as simple as plotting a nine month forecast (in fact, pregnancy is typically 40 weeks long, which is more than nine months!). Your doctor will give you your official due date after your first prenatal visit, but when you just gotta know the baby’s birthday now, count down to the big day with The Bump due date calculator.

If you conceived through in vitro fertilization (IVF), the pregnancy due date calculator may not be the right method for you. If you fall into that bucket, see below for how to calculate your due date for an IVF pregnancy.

How to Calculate Due Date By Conception

If you happen to know the exact day you conceived, you could calculate your baby due date by counting 38 weeks from the date of conception (38 weeks is said to be the length of human gestation). But figuring out an accurate due date based on conception date is easier said than done: Unless you were really deliberate about tracking your ovulation or were able to make a baby in one shot (and had no extracurricular fun with your man otherwise), knowing which day was the big one isn’t clear cut. Even if you had sex only once during your six-day fertile period, you may not have actually conceived that day unless you happened to be ovulating. Conception happens when your egg is fertilized, but since sperm can live inside your fallopian tubes for up to five days waiting for an egg to be released, you could conceive up to five days after having sex. It’s enough to make a mom-to-be’s head spin. That’s where a due date calculator, also known as a pregnancy calculator, helps.

How to Calculate Due Date Based on Last Period

The most common way of determining when baby will be born is to calculate your due date based on your last menstrual period (LMP), which is how a pregnancy calculator works. If you have an average-length cycle (about 28 days), your period came about two weeks before you ovulated and conceived. To arrive at your baby due date, count forward 40 weeks (that’s the 38-week gestation plus two weeks) from the first day of your last period and mark a big X on your calendar for Baby Day! Or, if you’d rather not rely on your math right now (the pregnancy brain struggle is real), simply plug the date of your last menstrual cycle into The Bump pregnancy due date calculator above and get instant results. Of course, not every woman has a 28-day cycle or ovulates exactly mid-cycle (and not everyone remembers with perfect precision the first day of their last period). Another (more accurate) way to calculate your pregnancy due date is by ultrasound (see below).

How to Calculate Due Date By Ultrasound

Other than using a due date calculator based on your LMP, your doctor can also calculate your pregnancy due date by ultrasound, especially if you have an irregular cycle. When you go in to see your OB or midwife for your first prenatal appointment, they’ll likely schedule you for an ultrasound to confirm the pregnancy. During the ultrasound, the technician or physician will measure baby’s length, from crown to rump, to gauge how far along you are and calculate your due date. According to the American Academy of Obstetricians and Gynecologists (ACOG), a first-trimester ultrasound is the most accurate method to establish or confirm gestational age. If baby is much bigger or smaller than expected for the gestational age based on your LMP, your doctor will likely change your due date based on what they see on the first-trimester ultrasound.

How to Calculate IVF Due Date

If you underwent in vitro fertilization, you have the benefit of knowing exactly when the egg was fertilized, so an IVF due date calculator can be very precise. Plus, you don’t have to worry about any changes to baby’s anticipated date of arrival as you might with a pregnancy calculator based on LMP. (Whether baby is actually born on that day is of course another story.) Your baby due date is determined by either when you ovulate—meaning your egg retrieval or intrauterine insemination (IUI) day—or whether you’re having a three- or five-day embryo transfer. For a day three embryo, count forward 263 days from the transfer for your due date; for a day five embryo, add 261 days. If you’re undergoing IUI, your due date is 266 days (or 38 weeks) later.

How to Calculate Due Date for Twins

If you naturally conceived twins or multiples and haven’t already been told your pregnancy due date by your doctor, you can use The Bump due date calculator above based on your LMP, just as you would if you weren’t doubling (or tripling) down on babies. How to calculate due date for multiples is the same method you’d use for a singleton baby, but keep in mind that delivery day recommendations may change: For example, di/di twins are usually delivered between 38 weeks and 38 weeks and six days instead of 40 weeks. So just know that twins may arrive earlier than the due date noted by the pregnancy calculator. A first-trimester ultrasound is the most accurate way to calculate due date, and changes to the date after that measurement is done are extremely rare.

Now that you know what month baby will be born in, you can also get the scoop (just for fun) on if your wee one will supposedly be a boy or a girl. Use the Chinese Gender Chart to predict baby's sex while you wait for the official (and more reliably accurate) ultrasound or prenatal testing results.

Medical content was reviewed August 2020 by Temeka Zore, MD, a board-certified ob-gyn and reproductive endocrinologist and infertility specialist at Spring Fertility in San Francisco.