The Truth About Pregnancy Due Dates

How to Calculate Your Due Date

Young man and pregnant girlfriend face to face and holding hands in field, waiting to meet their baby on the due date
Calculating your due date is more complicated than it appears. Cultura RM/Chev Wilkinson / Getty Images

Finally, you get a positive pregnancy test result! Your next question will likely be, “What’s my due date?” You want to know how long you have to wait to see this miracle.

You could use an online due date calculator. Most work by asking for your last menstrual period.

However, you can figure out your due date yourself, if you want. 

To do this in your head, just...

  1. Note the first day of your menstrual cycle this month
  2. Add seven days
  3. Now, move the month back three months
  4. Add one year

Then, you'll have your due date.

Here are some examples.

If your last menstrual period was 2/11/2015, first, add seven days: 2/18

Now, move back three months: 11/18

And, one year ahead is your due date: 11/18/2016

If your last menstrual period was 10/15/2015, first, add seven days: 10/22

Now, move back three months: 7/22.

Next, move one year ahead to get your due date: 7/22/2016

The average gestation for a singleton pregnancy is (generally) considered to be 280 days (or 40 weeks) from your last menstrual period. This method will give you the typical 40-week pregnancy.

However, due dates are more complicated and less certain than that.

In this article, you’ll learn...

  • Why due dates are more like due periods
  • Reasons you may deliver sooner or later than your due date
  • Why due dates are calculated by your last menstrual period and not ovulation day
  • Why ultrasound dating is even better – sometimes
  • Why none of this matters if you conceived with IVF
  • Why you need to care about your due date – even if it’s “just” a range

Keep reading for the answers...

Pregnancy Due Dates Are More Like Due Periods

Woman circling day on wall calendar, close-up
Think of your due date more like a due period. It's just a best guess. Jeffrey Coolidge / Getty Images

Your doctor will give you a due date – but the odds of you giving birth on that day are very low.

In fact, according to one study, only 4% of women give birth exactly 280 days (or 40 weeks) from their last menstrual period.

Just 70% will deliver within 10 days of their due date.

This is true even for due dates calculated via ultrasound, which is considered to be more accurate. (More on that soon.)

You should see your due date as a due period, with your due date somewhat in the middle.

You could deliver within a four to five-week range, and you wouldn’t be considered pre-term or post-term.

Why are due dates so inaccurate?

There are three big reasons.

One, most due dates are calculated based on the last menstrual period and assume you ovulated exactly 14 days later.

But many women ovulate sooner or later.

Also, implantation of the embryo also varies. The time of implantation can also affect how long you’ll be pregnant.

Even with fertility treatment, we don’t really know when you conceived unless you did IVF.

With Clomid or IUI, you might know when you had your trigger shot. You might know when you got a positive ovulation test result, or you might know when your basal body temperature chart indicated ovulation.

But even that isn’t 100% accurate. Studies have found that ovulation can occur a few days off from what a positive ovulation test or BBT chart indicates.

Secondly, we don’t know for certain how long human gestation is.

Primarily, this is because the moment of conception is hidden. It’s not an easy topic to study. With the exception of IVF, we have no idea when it happens.

Third, even the 40 weeks time period is debatable.

Some studies say 280 days (or 40 weeks) is the best guess for a due date. Other studies say 282 days (or 40 weeks and 2 days) is better.

Due dates are complicated.

On top of all this, there are a number of factors that may cause you to give birth sooner or later than average...

You May Deliver Sooner or Later Than Average if...

7 Pounds 12 Ounces
Believe it or not, your weight at birth may affect how long you'll carry your own baby. Layland Masuda / Getty Images

Two women get pregnant on the exact same day. But one gives birth earlier or later than the other. Why?

Well, we don’t always know why. However, according to the research, here are a few possible reasons.

Age: Generally speaking, women who are 35 years or older tend to give birth earlier or later than their due dates.

In fact, the older you are, the more likely you are to go over your official due date.

History of infertility: women who struggled to conceive are more likely to give birth early. They are at a higher risk of premature labor, which can be threatening to your baby’s health.

This is true even for singleton pregnancies. (You don’t have to have twins for your risk of premature labor to go up.)

The theory is that whatever hormonal imbalances caused the infertility also impact the hormones regulating pregnancy and childbirth.

Twins or More: most people know that the more babies growing in your womb, the more likely you’ll give birth early.

Twins tend to be born earlier than singletons. Triplets tend to be born earlier than twins. Quadruplets tend to be born earlier than triplets.

Your doctor may even give you two dates: a regular due date and a slightly earlier date, with the explanation that twins sometimes come sooner.

(This doesn’t mean your babies will be ready for the world at birth sooner. Just that you shouldn't be surprised if you go into labor earlier.)

How much you weighed at birth: yes, your birth weight can affect how long you are pregnant.

According to one study, the bigger your birth weight was, the longer you may carry your baby.

Researchers theorize that being bigger at birth might mean your uterus or pelvic capacity is also larger.

More room for the baby to grow, more time in the oven.

Later implantation: conception is when the sperm fertilizes the egg. This occurs when the egg is traveling down the fallopian tube. Eventually, the fertilized egg will find its way to the uterus and implant into the endometrium.

The time between fertilization and implantation varies between pregnancies. One study found that embryos that implanted later were more likely to have longer gestations.

Slow or abrupt increase in progesterone: the corpus luteum – which is formed from the follicle that released the egg – produces progesterone soon after ovulation.

When pregnancy occurs, pregnancy hormones signal the corpus luteum to continue to produce progesterone and increase those levels.

For some women, the levels rise rapidly. Others get a slow and later rise.

Research has found that women with the more sluggish progesterone rise tend to give birth earlier. 

Most Due Dates Are Calculated Based on Last Menstrual Period

Clock in front of a woman's abdomen
It's really impossible to know the exact moment of conception. by Christopher Wesser - / Getty Images

If you conceived on your own, your due date will be based on your last menstrual period (or LMP). This is also likely how your due date will be calculated if you took Clomid or fertility drugs.

Why are due dates based on LMP and not ovulation day? Wouldn’t ovulation day be more accurate?

The practical answer is because most women have no idea when they ovulated. But most know when their last period was.

But what if you do know when you ovulated? This is very likely if you’ve been charting your basal body temperature or if your doctor was monitoring your cycle, because of fertility treatment.

A good doctor will take into account your ovulation date. If you know you ovulated closer to Cycle Day 21, tell your doctor. That can push your due date a full week ahead.

(Just remember – whether you date your pregnancy on your last menstrual period or ovulation, your due date is still a range of days.)

However, there is a more accurate way to date a pregnancy than by calculating from ovulation. And that’s via ultrasound...

Ultrasound Is the Most Accurate for Pregnancy Dating

Pregnant woman having sonogram
Ultrasound is an accurate method of checking and calculating your due date. John Fedele / Getty Images

Dating pregnancy based on ovulation is more accurate than using your last menstrual period. But dating with ultrasound is even better.

It’s not usual for a woman to be given a due date, go for an early or routine ultrasound, and come out with a “new and improved” due date.

What happened?

Research has found that we can get a more accurate date by measuring how big the fetus is.

One method of dating is based on crown-to-rump measurements. Ideally, this should be done between 9 and 13 weeks (based on your last menstrual period.)

If the estimated ultrasound due date is less than 7 days different from the last menstrual cycle due date, your due date doesn’t change.

If it’s off by more than 7 days, then your due date may be adjusted.

What if you’ve passed 13 weeks?

There’s another ultrasound measurement that can be used to date the pregnancy. It’s known as biparietal diameter (BPD).

There’s some debate on what is more accurate – crown-to-rump measurements in the first trimester, or BPD measurements in the second trimester.

One study compared estimated due dates based on last menstrual period, crown to rump measurements, and biparietal diameter.

They looked at how many pregnancies went past the 41-week mark based on the various due date estimates.

  • 31% went past 41 weeks when using last menstrual period
  • 25.1% went past 41 weeks when using crown to rump length
  • Only 17.3% went past 41 weeks when biparietal diameter was used

Of course, your doctor should discuss any changes to your due date and why.

With that said, if you conceived with IVF treatment, your due date won’t change... ever.

If You Had IVF, Your Due Date Should Never Change

IVF treatment
Only during IVF treatment do we know the exact moment of conception. Science Photo Library / Getty Images

As mentioned earlier, it’s usually impossible to know exactly when conception took place.

That’s not the case with IVF.

IVF – which stands for in vitro fertilization – is literally translated as “fertilization in the lab.”

If you conceived with IVF, your doctor knows exactly when you conceived.

But IVF due dates aren’t calculated based on fertilization day. They are calculated based on the embryo transfer date.

If you had a Day 5 embryo transfer, your due date should be 261 days later.

If you had a Day 3 embryo transfer, your due date should be 263 days later.

Use this nifty IVF calculator to figure out your due date.

If a future ultrasound indicates a “wrong due date,” your due date will not change. More likely, your doctor will closely monitor the pregnancy for a while. The fetus may not be developing as expected.

But, you may be wondering, why does an accurate due date even matter?

If due dates are “due periods” anyway, why bother to shift them by one or two weeks? 

If Due Dates Are a Range, Why Does Accuracy Matter?

Caring doctor talking to pregnant woman
Discuss with your doctor any concerns or questions you have on your due date. Hero Images / Getty Images

Your due date isn’t set in stone. However, you still want that date to be as accurate as possible.

Important medical decisions are based on that date. Being a week or two off could lead to mistaken actions. Those actions can have serious consequences.

For example, if you go into labor early, your doctor will consider your due date before deciding to try and stop your labor or allow it to happen. They may also need to decide quickly whether to give you steroid injections to mature your baby’s lungs (if they think he’ll be born prematurely.)

If your doctor thinks you’re further along than you are, he may not stop labor when it should be stopped. If your doctor thinks you’re earlier than you are, you may receive medications that are unnecessary.

Going past your date is also an issue. You might think that the longer the baby “cooks”, the better.

However, there can be complications to going post-dates. For one, the placenta can start to break down.

If your doctor thinks you’re one or two weeks post dates, when you’re actually right on time, he may schedule an induction.

In the worst case, your baby may not be ready for the world. At best, even if your baby is ready, you and your baby will go through the stress of an induction. That comes with risks, to both of you.

A due date isn’t just “for fun.” That date is used to make important decisions.

This is why you should discuss your due date with your doctor. If you went through fertility treatment, make sure your OB/GYN has all your treatment dates. If your doctor wants to change your dates, ask why.

Your health and your baby’s health may depend on it.

More on pregnancy after infertility:

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