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:

You are weeks pregnant!

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pregnant woman predicting due date

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. The following two methods are utilized in our due date calculator:

  • 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 (which would also be your conception date) – 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.

IVF Transfer Date

If you’re an IVF Mama-To-Be:

  • IVF 3-Day Transfer Date – You can calculate your pregnancy 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.

Ultrasound Scan

Rather than using an LMP pregnancy due date calculator, you can also seek an ultrasound scan with a doctor. This is also an efficient method if your menstrual cycles are irregular. The ultrasound technician will be able to measure the average length of your baby, your abdominal circumference, and other biometric variables to anticipate how far you have until your due date. This method could accurately determine the fetus’s gestational age and provide you with a better estimated due date.

While all pregnancies have due dates, this is simply an estimated date and 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 and estimates of a baby’s development.

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 provide your baby’s estimated birth date:

  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, thus, neither is a baby’s birth. 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 expecting moms or their child. This can significantly alter the timeline of your baby’s arrival.

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

Changing Due Dates

During routine ultrasounds in your first and second trimester, your doctor may change your due date based on what they see. Reasons why your due date may change:

  • 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.
  • Irregular periods
  • Your fundal height (size of uterus) is abnormal
  • You had an ultrasound during the second trimester
  • Abnormal alpha-fetoprotein levels (proteins produced by your growing baby)

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
    • Receive proper prenatal care The SneakPeek Early Gender DNA Test is clinically-proven to be 99.9% 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!


Methods for estimating the due date”: “4 Major Reasons Why A Woman’s Pregnancy Due Date Changes” by Bianchi Mendoza

ACOG. Definition of Term Pregnancy.

CDC. Births: Final Data for 2017.

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