Pregnancy Due Date Calculator: When Is Your Baby's Expected Due Date?

by Team Scary Mommy
Originally Published: 
pregnancy due date calculator
Kwangmoozaa/ Getty

Congrats, mama! If you’re here, it means you’ve got a bun in the oven — or some early pregnancy symptoms have you pretty darn sure you do. We don’t have to tell you how exciting this is (you may already be looking up baby names!). But that doesn’t mean we don’t have any sage words for you. In addition to the congratulatory sentiment we started with, we can help you home in on your baby’s anticipated arrival with Scary Mommy’s handy-dandy due date calculator.

That’s right! You can use our pregnancy due date calculator, below, to see when your baby will make their big debut into the world and find out just how many weeks pregnant you are. (Feeling playful? See what the ancient Chinese gender predictor thinks you’re having.)

[due_date_calculator padding_y=45]

How is due date calculated?

There are three main ways doctors (or you, with the help of this calculator) determine when your little nugget will tentatively enter the world. Per the American Pregnancy Association, the technical term for this — meaning the age of your baby — is gestational age.

1. First Day of last period

One way to figure out gestational age is by calculating from the first day of the mother’s last period. So, since most pregnancies last around 40 weeks, you can typically pinpoint a due date by counting 40 weeks, or 280 days, out from the first day of your last menstrual cycle. Or, if you prefer, you could subtract three months from the first day of your last menstrual cycle and add seven days to the resulting number.

People undergoing intrauterine insemination (IUI) procedures should calculate their due date using the natural conception method, using the date of their last period. This is because the IUI procedure occurs during ovulation.

2. Conception date

Having said all of that, this is not a perfect science. Not all women have regular menstrual cycles. Also, some of us can’t remember what day it was yesterday, much less the first day of our last period (even if we know we should be keeping tabs).

In these cases, it might work better to determine your due date by conception date. You might remember it because, well, what led to it is a little more fun-slash-memorable than the first day of your period. From there, you just add 266 days to get your tentative due date since conception is generally about two weeks after the first day of your last period.

3. IVF transfer date

IVF, or in vitro fertilization, is the process of extracting eggs, retrieving a sperm sample, and then combining the two in a lab to form an embryo. The embryo is then transferred to the uterus. Granted, that’s an extremely simplified version of the events leading up to and during IVF, but you get the gist.

So, with IVF, you can use two methods to calculate your due date: the egg retrieval date or the embryo transfer date. The egg retrieval date works if a woman undergoes IVF using freshly harvested eggs. In that instance, due date can be estimated by counting out 38 weeks from the day the eggs were harvested. If the woman underwent IVF with frozen embryos (which is common), the due date can be calculated using the date of the embryo transfer.

Why do you add two weeks to pregnancy?

During that first month of pregnancy, you’ll undoubtedly feel a lot of things — confused might be one of them. One reason? Even though you ovulated and conceived only two weeks ago, you’ll be considered four weeks pregnant. In the first two weeks of that, you aren’t actually pregnant. Your body is preparing for ovulation as usual. But the development of pregnancy is counted from the first day of the last menstrual period, which includes that time.

Can you plan your due date?

There are myriad reasons to want to pick your own due date. Maybe you’re really into astrology and there are certain zodiac signs you do not want to give birth to. Or perhaps you want to time it so you aren’t super-pregnant in the sweltering heat of summer. Perhaps you’re a teacher and you want to time it with your summer break. Ay, we get it.

If you just want to ballpark it without having to do the math yourself, you could always plug “reverse due date calculator” into Google. Just go in knowing you should probably take the results with a grain of salt. There are too many factors to take into consideration to be able to neatly nail down the exact time to get knocked up so your baby can have a specific birthday. Then again, hey, you may get lucky (on top of “getting lucky,” no less).

How certain are due date predictions?

If there’s one thing you can count on about having a baby, it’s that you can count on the unexpected. You might not find out your baby won’t be born on their due date until they arrive early or, bless your soul, late. Or your doctor might adjust the due date during your pregnancy based on baby’s measurements. Well, only 1 in 20 women actually delivers on her due date. So, there’s that.

How can you tell you conceived?

Although some pregnancy symptoms can start very early — and, again, the experience can differ from person to person — you may very well not notice anything different in those first few weeks. An estimated 60 percent of pregnant people begin to experience some early pregnancy symptoms (e.g., nausea, tender breasts, fatigue, headaches, food aversions) around five or six weeks after their last menstruation, or two weeks from a missed period. The best way to know you’ve conceived is to have a pregnancy ultrasound, which brings us to our next point.

You’re pregnant; now what?

The first thing you should do after finding out you’re pregnant is to find an obstetrician you like, who comes highly recommended, whose ideology and birth approach meshes with yours. If you have a preexisting medical condition or have experienced a high-risk pregnancy in the past, or are expecting multiples, you may need to find a maternal-fetal medicine specialist or perinatologist. Perinatologists are obstetricians and gynecologists who deal with high-risk pregnancies.

Next, schedule your first appointment. If this is your first pregnancy, the OB may not see you until either you are six weeks or eight weeks pregnant. If you’ve experienced an ectopic pregnancy in the past, you should be seen right away, as the chances for another ectopic pregnancy are higher for women who have had one before. This is also a time to be mindful of any bleeding you may experience as most miscarriages and chemical pregnancies occur in the first trimester of pregnancy, and especially in the earlier weeks.

However, not all spotting is created equal. Some women may notice light spotting during implantation bleeding right around the time they would expect their period. Implantation bleeding occurs when a fertilized egg attaches to the uterine wall, causing light spotting or bleeding around 10 to 14 days after conception.

How long is pregnancy, and how is it broken up?

If this is not your first pregnancy, you probably already know that the 40 weeks of gestation are broken up into three trimesters. Though some obstetricians can go back and forth on the weeks, the three trimesters are broken up like this:

  • The first trimester is weeks 1 through 13.
  • The second trimester is weeks 14 through 27.
  • The third trimester is weeks 28 through 40.

How was the due date calculated in the past?

Here’s where modern science comes in clutch. The reality is some women didn’t know they were pregnant until they either missed quite a few periods or their belly popped. That’s not to say women were completely flying in the dark during pregnancy. While German obstetrician Franz Naegele came up with Naegel’s Rule resulting in the 280-day gestational model we use today, the ancient Greek philosopher Aristotle documented the average human pregnancy was 10 lunar months. With a lunar month spanning 28 days, that figure added to 280 days exactly.

Aristotle didn’t discover something revolutionary, even for his time. Ancient civilizations have used the lunar calendar to date pregnancies for centuries, counting roughly 10 moon cycles as a full-term pregnancy and working backward to the woman’s last menstruation cycle.

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