Pregnancy Due Date Calculator

Babies are unpredictable. This applies to their due dates too. My son’s doctor-estimated due date was July 11, and the scheduled C-section to get him out was supposed to be July 6—but I started having contractions during a fireworks show on July 4. I got excited that he had decided to be born on the Fourth of July—baby, you’re a firework!—and then his birthday ended up being July 5. In the end, it all worked out. (It’s probably for the best for him not to share his birthday with a huge holiday!)

Once you find out you’re pregnant—congrats!—the next question on your mind might be, “When is my due date?” or “How far along am I?” Your doctor will give you your estimated due date after your first prenatal visit, but keep in mind that it’s just that—an estimate. “About 4 to 6 percent of babies are born on their actual due date,” points out Christine Greves, MD, an ob-gyn at the Orlando Health Women’s Institute.

That said, if you just gotta know baby’s birthday now(!), you can use The Bump due date calculator below to calculate their approximate due date based on the first day of your last menstrual period (LMP). You’ll also find out how to calculate your due date for in vitro fertilization (IVF), for a twin pregnancy and more.

Key Takeaways

• What is a due date? A due date is the approximate date that baby’s expected to be born. Only about one out of every 20 babies is born on their actual due date.
• How far along am I? The most common way to estimate a due date is by noting the first day of your last menstrual period (LMP) and adding 280 days.
• Calculate due date: You can enter this into The Bump due date calculator tool and find out your estimated due date.
• Dating ultrasound: If you have an irregular cycle or don’t know when the first day of your LMP was, your provider can confirm or adjust your due date with an early-pregnancy ultrasound. These early ultrasounds are typically more accurate than LMP-based measurements, so if there’s a large discrepancy between these, your provider will likely adjust your due date.

How to Use The Bump Due Date Calculator Tool

Know the first day of your last menstrual period (LMP)? Enter the date into the due date calculator tool above, and—voilà!—your due date will appear. Keep in mind that this is based on a typical 28-day cycle—and not every woman has a 28-day cycle or ovulates exactly mid-cycle.

What Is a Due Date?

A due date in pregnancy is the estimated date that baby’s expected to be born. A typical pregnancy lasts 280 days, or 40 weeks—which translates to over 9 months (9.7, to be exact). Your ob-gyn or midwife may have pulled out a pregnancy calculator chart like the one below to calculate your due date. You, too, can calculate due date by adding 280 days to the first day of your LMP.

Image: The Bump

How accurate are due dates?

Suffice it to say, they’re not that accurate. Only about 4 to 6 percent of babies arrive on their actual due date. There are just too many factors that play into this. “I always tell people to think of their due date as the center point of a five-week window—not a deadline or a prediction,” says Shalini Shah, IBCLC, a lactation consultant, full-spectrum doula and guide with Millie women’s health clinic. “Your body doesn’t know what day it is, and labor follows its own timeline.”

How to Calculate Due Date

You’re likely trying to answer the burning question, “When is my due date?” Again, the most common way to compute pregnancy due date is using the first day of your last menstrual period (LMP), just as you did in the above due date calculator, says Shah. But, “That assumes you ovulated on day 14 of a 28-day cycle, which isn’t true for everyone,” she adds. See below for more information on how to calculate your due date.

First day of last menstrual period

Again, the most common way of determining when baby will be born is to calculate baby’s due date based on your 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 due date, count forward 40 weeks (that’s the 38-week gestation plus two weeks) from the first day of your LMP and mark a big X on your calendar for Baby Day! Again, you can also try plugging the start date of your LMP into The Bump pregnancy due date calculator above to get instant results.

Remember, while a 28-day cycle is considered the norm, it can vary—and not everyone ovulates exactly mid-cycle. (Additionally, not everyone will remember with perfect precision the first day of their last period).

Conception date

If you happen to know the exact day you conceived, you could calculate baby’s due date by counting 38 weeks from the date of conception (38 weeks is said to be the length of human gestation), according to Cleveland Clinic. 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 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 sperm and egg join, after which an 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 your head spin. (That’s why The Bump due date calculator, or a pregnancy calculator, can help clear things up!)

IVF transfer date

If you underwent in vitro fertilization (IVF), 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 your LMP. (Whether baby is actually born on that day is, of course, another story.)

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.

Early ultrasound dating

Other than using a due date calculator based on the first day of your LMP, your doctor can also calculate your pregnancy due date by ultrasound, especially if your cycle is shorter or longer than 28 days. “If you have irregular cycles or you’re unsure of your dates, your provider will usually confirm (or adjust) the due date with an early ultrasound,” says Shah. 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 may change your due date based on what they see on the first-trimester ultrasound.

Your Pregnancy Timeline

Now that you’ve calculated your estimated due date, you’re likely wondering what the next 40-or-so weeks of pregnancy will entail. Here’s a rundown of what pregnancy milestones you should expect, by trimester, as you look forward to your due date.

First trimester

Congratulations! Getting that positive pregnancy test is a huge milestone in itself. The first trimester of pregnancy lasts from week 1 (when you’re not technically pregnant yet) to week 13. The first trimester is marked by less-than-pleasant early pregnancy symptoms such as morning sickness, constipation and fatigue. Make sure to stay hydrated, start a prenatal vitamin (if you haven’t already) and eat a balanced pregnancy diet.

During the first trimester, it’s important to schedule your first prenatal visit—around 8 weeks—and keep up with your regular appointments. At your 8-week ultrasound, your provider will look for a fetal heartbeat, as well as determine baby’s due date. Next, you’ll need to schedule the first trimester screening between weeks 10 and 13, which will include a nuchal translucency ultrasound. You may also choose to do non-invasive prenatal testing (NIPT) after 10 weeks, at which point you can find out baby’s sex. By the end of your first trimester, several of baby’s major organs will have started to form, and you’ll be able to see baby via ultrasound (exciting!).

Second trimester

The second trimester of pregnancy lasts from week 14 to week 27. Your morning sickness should start to subside by this point, and you may feel less tired—which is why this is known as the pregnancy sweet spot! You’ll still experience symptoms, though: Back pain, leg cramps and skin changes like the linea nigra are all pretty common. Now’s the time to take advantage of your energy: Start a pregnancy exercise routine, research childcare options and create a baby registry. This is also a great time to work on your birth plan!

Baby’s growing by leaps and bounds during the second trimester, and your pregnant belly is becoming more and more visible. At your 20-week ultrasound, your healthcare provider will give you a complete overview of baby’s growth and development. The scan also looks at baby’s sex (if you haven’t found out already), as well any possible anatomical or chromosomal abnormalities. Another important second-trimester test is the glucose challenge screening between 24 and 26 weeks; this screens for gestational diabetes.

Third trimester

Welcome to the final stretch! The third trimester, lasting from 28 weeks pregnant to when you give birth, is when you’ll get (much) bigger, and experience symptoms such as heartburn and having to pee all time (even in the middle of the night). Baby’s organs and systems will continue to mature and develop until they’re ready to enter the world. At 40 weeks, baby’s as big as a watermelon! This is the time to choose a pediatrician, pack your hospital bag, hire a doula if you’re planning on it and stock up on postpartum recovery supplies.

Your prenatal visits will be more frequent now, and you’ll be seeing your OB every two weeks or so between weeks 28 and 36, and once a week after 36 weeks pregnant. Your provider will perform the Group B strep test between weeks 36 and 37. During the last few weeks, your provider will perform an ultrasound to look at baby’s positioning, do a pelvic exam to evaluate cervical effacement, and dilation and discuss your birth plan. After that—it’s go time!

How Accurate is a Due Date Calculator?

A due date calculator will show you baby’s approximate due date based on the first day of your LMP. It’s used as an estimation tool—but it’s still super helpful for you and your provider in order to track baby’s growth and your pregnancy.

Plus, more from The Bump:

Can You Get Pregnant on Your Period?

Calculating Your Fertile Window: When Are You Most Fertile?

The Bump Ovulation Calculator