Due Date Calculator

What is a due date calculator?

A due date calculator helps you estimate when your baby will be born. By calculating your due date, you’ll have an idea of your baby’s birthday. But keep in mind that your due date is only an estimate. Only about 4% of babies are born on their due date.

Knowing your due date is important for several reasons. It helps you plan and prepare for your baby’s arrival. It also gives your pregnancy care provider a roadmap to monitor your pregnancy, helping them figure out when certain milestones should occur and when they should perform tests.

You can calculate your due date at home using a couple of different methods. But these methods aren’t always accurate. When you visit your provider, they may give you a different due date based on the size of the fetus on ultrasound and the date of your last menstrual period.

What is a due date?

It takes about 38 weeks (266 days) for a fetus to fully develop. Your due date is the day you’re expected to give birth based on this 38-week fetal development process.

But keep in mind, babies can be born at any time, and many factors play a role in determining when you go into labor and when your baby will be born.

How do I calculate my due date?

There are two main ways to calculate your due date:

• The first day of your last period.
• The date you conceived.

The date of your last period

You can calculate your due date if you know the first day of your last menstrual period (LMP) before you got pregnant. Your due date is 40 weeks after the day you started your last period. This is the most reliable way to calculate your due date on your own at home, but it still isn’t perfect.

It works well because many people know the day they started their last period, either because they track it on an app or because they remember once they look at a calendar. But it might not always be accurate, especially if you have an irregular menstrual cycle or are on birth control.

How do I calculate my due date if I don’t know the date of my last period?

Don’t worry — not knowing the date of your last period doesn’t mean you won’t get a due date. Your pregnancy care provider will perform an ultrasound to determine how far along you are in your pregnancy and give you a due date. The due date your provider gives you is the most accurate.

The date you conceived (got pregnant)

If you know the exact day you conceived, you can use that information to figure out when your baby is due. Count ahead 38 weeks from the day you got pregnant. That day is your due date. This method is much harder to use and much less accurate because you must be 100% sure what day you got pregnant.

You may only know the specific day you got pregnant if you know when ovulation occurred. But even if you know when you ovulated, you still may not know exactly when pregnancy occurred. This is because sperm can live inside your body for up to five days and the entire process of releasing an egg lasts about 24 hours. It can be very hard to pinpoint the exact day the sperm and egg officially got together.

How do I calculate my due date if I had IVF?

If you did in vitro fertilization (IVF) to get pregnant, you can use your IVF transfer date to calculate your due date:

• If your provider transfers the embryos five days after egg retrieval, count ahead 261 days from the transfer date.
• If your provider transfers the embryos three days after egg retrieval, count ahead 263 days to determine your due date.

Like the other methods for calculating your due date, it’s always best to let your provider confirm your due date.

How accurate is a baby due date calculator?

A due date calculator is only an estimate. Your healthcare provider uses ultrasound to confirm your due date, typically at your first prenatal appointment. During an ultrasound, your provider measures the fetus. These measurements help your provider determine gestational age and confirm your due date. An ultrasound is the most reliable way to determine a baby’s due date.

Why is it important to calculate my due date?

It’s important to know your due date for planning purposes — knowing your expected delivery date helps you prepare to welcome your baby. You’ll have time to pack for the hospital and get the things you need for the baby, like a car seat. If you have other children, counting down the days until the new baby arrives can help siblings prepare for a new family member.

Knowing when you’re due also allows you to get appropriate prenatal care. This includes taking prenatal vitamins and getting prenatal screenings at the right times during your pregnancy. Your provider will also measure the fetus’s size and position throughout your pregnancy to ensure that it’s growing and developing as it should.

What is a reverse due date calculator?

A reverse due date calculator is helpful if you already know your due date and you want to find out when you got pregnant. To find out when you conceived, just subtract 266 days from your expected delivery date. You probably conceived around that day.

What do I need to know about using a pregnancy due date calculator?

A due date calculator may not be completely accurate. This is especially true if you aren’t exactly sure when you had your last period, you have irregular periods or you don’t know when you conceived. You should see your pregnancy care provider for a more precise test, such as an ultrasound, to confirm your due date. Use the due date your provider gives you as your official due date.

Can my due date change?

Yes. Due dates are only an estimate. When you see your provider for an ultrasound, they may tell you that your due date has changed. Sometimes, fetal measurements show that you aren’t as far along as you thought. Or, if the fetus is bigger than expected based on your due date, your provider may adjust your due date to make it a few days earlier. This is all perfectly normal.

Providers consider a pregnancy to be full term after 37 weeks. So, keep in mind that you may very well deliver your baby before or after your due date.

How does carrying multiples affect my due date?

Expecting twins or triplets is different from expecting one baby for many reasons. If you’re pregnant with multiples, your pregnancy care provider will use your last menstrual period and an ultrasound to determine your due date. But this is where it gets tricky. Pregnancy care providers typically want to deliver multiples right around 36 or 37 weeks. So, while your due date is calculated similarly to a singleton pregnancy, your pregnancy care provider will likely tell you to expect your babies to come sooner. 

What is Naegele’s rule in pregnancy?

Naegele’s rule is a way to calculate your pregnancy, assuming you have a 28-day menstrual cycle. It involves:

• Adding seven days to the first day of your last period.
• Subtracting three months from that date.
• Adding one year to that date (if necessary).

For example, if your last period started on February 1^st, you’d add seven days and get February 8^th. Then you’d subtract 3 months from February 8^th. Your due date would be November 8^th.

How am I four weeks pregnant if I conceived two weeks ago?

Calculating weeks of pregnancy is weird because you’re “pregnant” longer than it makes sense to be. The reason you’re already four weeks pregnant when you miss your period is because pregnancy is calculated from your last menstrual period. So, since a menstrual cycle is roughly four weeks long, you’re already four weeks pregnant when you miss your period.

A note from Cleveland Clinic

Finding out your baby’s due date is one of the most exciting parts of pregnancy. Once you have it, you may go into planning mode to prepare for your baby’s arrival. Your due date is important to your care, too — it allows your pregnancy care provider to give you the best care based on how far along you are in pregnancy.

While you can calculate your due date on your own, your pregnancy care provider gives you the most accurate due date. They use ultrasound to measure the size of the fetus and calculate a date, so their calculation may be different from yours. Your provider may even change your due date during your pregnancy, but it usually only changes by a few days.

And remember, a due date is just an estimate. While it gives you a fun ending point, the likelihood of your baby coming on its due date is low.