Pregnancy Due Date Calculator

Congratulations on your pregnancy! Ready to find out when your baby will arrive?

Calculation Method
First Day of Last Period

Your Due Date is Approx:

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Pregnancy | The Trimesters

Most pregnancies will last about 9 months, or 40 weeks. This timeline is broken up into 3 trimesters:

  • First Trimester – Weeks 0-12 – During this time, your baby will go from just a zygote (a fertilized egg) to an over-2-inch tall fetus. At this point, your baby has already developed the foundation for all of his or her major organs and systems.
    • During this time, your body is processing a major shift. Hormonal changes may cause morning sickness, frequent urination, and other
      bodily discomforts.
  • Second Trimester – Weeks 13-28 – Welcome to the baby bump trimester. During these next 16 weeks, your baby will be developing hearing, hair, eyebrows, and martial arts skills (many women experience the first kicks and movements between weeks 16 and 25).
    • Many consider the second trimester the “golden age” because of the reinvigorated energy many women experience. The negative
      symptoms of the first trimester will lessen and your belly will start to rise with a cute baby bump.
  • Third Trimester – Weeks 29-40 – The last trimester is marked by your baby’s final developments. His or her eyes will be able to open and close and even follow a light. Their lungs are nearing completion, and you both are gaining weight rapidly (this is a good thing!).
    • The light at the end of the tunnel approaches quickly. In the last month, you may experience trouble sleeping, lots of movement from
      your baby, and contractions (which can signal either real or false labor, called Braxton-Hicks).

You can use our Pregnancy Calculator to keep track of what week and trimester you’re in, so that you’ll always be prepared for what’s coming next.

Pregnancy | The Terms

At 40 weeks gestation, your pregnancy is considered “full term,” and it begins from the first day of your last period. But there are multiple ways to calculate your due date, and there are other “terms” you should know about.

According to the American College of Obstetricians and Gynecologists:

  • Early Term is a gestation period between 37 weeks – 38 weeks and 6/7 days
  • Full Term is a gestation period between 39 weeks – 40 weeks and 6/7 days
  • Late Term is a gestation period between 41 weeks – 42 weeks and 6/7 days
  • Postterm is when the gestation period is longer than 42 weeks

Methods of Due Date Calculation | How Our Pregnancy Calculator Works

There are four methods used to determine your due date. You can choose the one that you know best.

  • Last Menstrual Period – The most common method is calculating the due date based on the first day of your last period. Simply add 40 weeks, or 280 days, to that date—and voila! This is your due date.
  • Date of Ovulation – If you know your ovulation date, you can also calculate your estimated due date by adding 266 days to it. Most ovulation periods begin 14 days after your last period, that’s why these two methods are very similar.

If you’re an IVF Mama-To-Be:

  • IVF 3-Day Transfer Date – You can calculate your due date by adding 263 days to the last day of your IVF transfer date.
  • IVF 5-Day Transfer Date – You can calculate your due date by adding 261 days to the last day of your IVF transfer date.

While all pregnancies have due dates, few births actually occur on that exact day. We’ll dive into due date accuracies and reasons behind why your due date might change below.

Accuracy of Due Date Estimations

As mentioned above, not all pregnancies are delivered on their given due date. Because we’re talking about a human life developing here, dates and milestones are often estimates based on averages.

You may find this shocking; according to the CDC:

  • About 57% of deliveries take place between 39- and 40-weeks gestation (full term)

What’s so shocking about this?

To break this down further, that means each day has roughly a 4-5% chance of being the date of delivery. Because due dates are estimates based on the first day of the last period, that means only about 5% of babies arrive on their exact due date.

This doesn’t mean a pregnancy calculator isn’t helpful in preparing you for the unexpected.

But the question still stands: why are due date estimations… only estimations?

Why Pregnancy Calculators Can Only Estimate

There are few reasons to explore about why pregnancy calculators can only estimate:

  1. Calculation by ‘last menstrual period’ is based on a regular 28-day menstrual cycle, with ovulation occurring 14 days into the cycle. Thus, discrepancies exist because this doesn’t account for cycle length irregularities and variability in ovulation timing.
  2. The development of human life isn’t programmed. A growing baby is both art and science. While there may be trends researchers can point to (developing hearing around week 18, for example), the exact time at which babies develop is variable.
  3. Pregnancy complications can change the delivery timeline. Sometimes labor is induced to prevent harm to either mother or child. This can significantly alter the timeline of your delivery.

Changing Due Dates

During routine ultrasounds in your first and second trimester, your doctor may change your due date based on what they see. If your baby is noticeably smaller or larger than “the average,” then the doctor may be inclined to change the due date to fit their best estimations.

You Used The Pregnancy Calculator… Now What?

Great question. Now is the time to start preparing. Whether you’re in your first trimester or third, it’s never too late to organize your life, educate yourself on what to expect, and prep for the new addition to your family.

    • Visit your doctor – After the initial rush of finding out you’re pregnant, set up an appointment with your OBGYN to start monitoring your pregnancy.
    • Discover your baby’s gender – As early as 8 weeks into pregnancy, the SneakPeek Early Gender DNA Test allows mothers to discover their baby’s gender. Knowing whether you’re having a beautiful baby girl or boy can help build a connection between you and your child.
      • You’ll be able to start thinking of names earlier
      • You can throw a gender reveal party and let family and friends know
      • You’ll be able to create the perfect nursery

The SneakPeek Early Gender DNA Test is clinically-proven to be 99.1% accurate in determining babies’ genders at 8 weeks.

Use the SneakPeek Pregnancy Calculator to find out your due date and start tracking your pregnancy today!


ACOG. Definition of Term Pregnancy.

CDC. Births: Final Data for 2017.