Due Date Calculator

By SheCares Editorial Team | Updated: Jul 01, 2019

To some expecting mothers, it might sometimes seem like an eternity from receiving pregnancy confirmation to actual childbirth, making women eagerly await their due date.

Although studies have shown that no more than 5% of women give birth on their estimated due date, having an accurate delivery date is not only important for pregnancy logistics, like preparing the nursery or planning the last getaway, but also a reliable pregnancy due date ensures that women receive proper obstetric care and timely administered prenatal testing, both of which guarantee the best pregnancy outcomes.

Keep on reading to learn more about pregnancy calculators, including what they are, how an estimated due date is calculated, and how accurate are the estimations from due date calculators.

Pregnancy due date

Due Date Calculator

Calculation Method
First Day of Last Menstrual Period
How long is your cycle?

What is a Due Date Calculator?

Pregnancy due date calculator

A due date calculator, or a pregnancy calculator, is an internet-based tool that is used to obtain the baby's estimated date of delivery (EDD).

These estimations are based on the average duration of pregnancy, which is 40 weeks, or 280 days, although a healthy and normally developing pregnancy can last anywhere from 37 to 42 weeks.

A number of free pregnancy due date calculators is offered online by portals focused on pregnancy and female health. Besides an estimated due date, these calculators often make references of the baby's weight or size at various pregnancy stages.

How to Calculate Pregnancy Due Date?

How to calculate pregnancy due date?

There are three main ways through which an estimated due date can be obtained. They include the following:

Due Date Based on Last Period

The most common initial method of calculating the baby's due date uses a woman's last menstrual period

It consists of adding 40 weeks, or 280 days, to the first day of the last menstrual period (LMP). This method is the most accurate for women with regular, 28-day menstrual cycles with ovulation taking place on Day 14. It is worth noting that in the first two weeks of gestation, a woman is not actually pregnant yet.

Due Date by Conception

Another possible method of obtaining the pregnancy due date is based on the conception day.

It involves counting 38 weeks, or 266 days, from the day a woman has conceived. For this method to be accurate, a woman has to be tracking ovulation since conception can only occur in that 24-hour window since egg release. As such, most women find it easier to recall their LMP rather than their ovulation.

Due Date by Ultrasound

Also called a dating scan, an ultrasound in the first trimester is the more accurate method of establishing the baby's gestational age and the respective pregnancy due date, especially in the case of women not knowing their LMP as well as those with irregular periods, irregular ovulation, or suspected pregnancy complications.

From the fifth week of gestation, the first structures of the developing embryo can be monitored through a transvaginal or transabdominal ultrasound. Assessing fetal development, particularly crown-rump length (CRL), gives a precise gestational age needed for estimating the baby's due date.

How Accurate are Pregnancy Due Dates?

Being the most accurate due date predictor, a first trimester ultrasound has a 5-7 day error margin; however, its accuracy decreases with time.

As such, the first pregnancy due date estimated from a woman's last menstrual period might be adjusted based on the findings of the first trimester ultrasound. However, second and third trimester ultrasounds are not usually considered for making pregnancy due date adjustments.

How accurate are pregnancy due dates?
  • If due date obtained via first trimester ultrasound is within 5-7 days from LMP due date, the LMP due date should remain unchanged.

  • If the first trimester ultrasound differs from LMP due date by more than 7 days, the new pregnancy due date will be changed to the one obtained via ultrasound.

  • If the first ultrasound was done in the second trimester, and it differs from LMP due date by more than 7 days, its findings should replace LMP due date.

Key Takeaways

Pregnancy due date is one of the first and most important measurements taken in early pregnancy. Upon confirming that a woman is expecting, the expected date of delivery can be obtained with a due date calculator or manually by adding 40 weeks to the first day of a woman's last menstrual period. However, this method has its limitations as it applies to women who have 28-day cycles and regular ovulation taking place on Day 14. Aspiring mothers trying to conceive can also add 38 weeks to their conception date to estimate their pregnancy due date as long as they have been keeping track of their cycles with a fertility chart or ovulation trackers. Yet, the most accurate due date predictor is an ultrasound, which can measure fetal growth and produce gestational age. However, for an ultrasound to be precise, it should be done in the first trimester as its accuracy decreases as pregnancy progresses.