Pregnancy Due Date Calculator: Planning Your Baby’s Arrival

Medically reviewed by Dr. Amelia Grant, MD, FACOG, Board-Certified Obstetrician & Gynecologist
Written by Natalie Healey, Health & Pregnancy Writer

For our due date calculator, we’ve taken into consideration the different dates that can help estimate your baby’s due date. So, what you do is select or input the required data in the fields provided by our due date predictor.

The calculator processes the information instantly and displays the estimated due date. It also gives you a few other details on your baby’s development, so you can track your pregnancy progress and happily plan your baby’s arrival ahead of time.

There are four different methods you can use to calculate your estimated due date: 1) The last period date, 2) The conception date, 3) The IVF transfer date, and 4) The ultrasound scan method.

Last Period

Calculating your pregnancy due date using the last menstrual period method uses what they call Naegele’s formula, named after the German obstetrician who discovered it in the 19th century.

The obstetrician found that pregnancy lasts 38 weeks on average, which adds up to 40 weeks from the first day of your menstrual period.

So, to calculate your baby’s due date with this formula, you start by identifying the date of the first day of your last menstrual period and then count forward by 40 weeks (280 days).

This is practically done using the formula: First day of LMP + 7 days - 3 months + 1 year, which translates to these steps:

1. Identify the first day of your last menstrual period, say 18th August 2025.
2. Add 7 days, which gives you 25th August 2025.
3. Subtract 3 months, and you get 25th May 2025.
4. Add one year, which comes to 25th May 2026

So, your baby’s estimated due date is 25th May 2026.

This formula assumes a standard 28-day cycle. So, some experts suggest adjusting it case by case by adding or subtracting the difference in days of a longer or shorter cycle. This can make the method a bit complicated.

For a quick solution, feed the date of the first day of your last menstrual period into our baby due date calculator. Then specify in the field below if you have a 28-day cycle or a shorter or longer one. Your pregnancy’s due date will be generated automatically.

Especially if you have an irregular cycle, in which case the last period method is less precise, the ultrasound scan method is the way to go.

Your obstetrician or a sonographer measures the dimensions of your fetus and then compares the measurements to standard population data to determine the gestational age of your baby and, from that, the expected due date. 

This method is most precise in the first trimester. At this stage, the ultrasound measures the fetus’s length from head to bottom, also called the crown-rump length (CRL).

To use our due date calculator for this method, feed the ultrasound-calculated gestational age of your baby (in weeks) and the date the ultrasound was done into our due date predictor to generate the EDD.

You may already know the due date because maybe you’ve already visited your obstetrician and done an ultrasound or have manually calculated the due date from the LMP date, the IVF transfer date, or the conception date. Even then, our estimated baby due date calculator can still be helpful in the following ways:

• Serves as a pregnancy tracker to monitor the progress of your pregnancy week by week. This helps you know how far along you are and how many weeks are left until your baby’s birth.
• From the additional info generated alongside the due date, you can check the main development and growth milestones of your baby week by week and know when a trimester starts and ends.
• You can plan your prenatal appointments, maternity leave, and other details around your baby’s birth ahead of time.

All this helps you stay closely connected to your unborn baby throughout the pregnancy period.

Yes. Your due date can change in certain situations.

First, it’s important to keep in mind that most of the methods for calculating the expected date of delivery provide an estimate. In fact, data shows that only about 4-5% of LMP due date estimations are exact.

For this reason, your obstetrician can adjust the date in the following cases:

• Your estimated due date using the LMP or conception date method does not match the ultrasound scan date. A study found that over 40% of expectant mothers had their LMP and conception date estimates adjusted at the first-trimester ultrasound scan because they showed a discrepancy of more than 5 days. 
• You do not have a regular 28-day cycle. This means your ovulation can be earlier for shorter cycles or later for longer cycles, predicting an earlier due date in the first case and a later date in the second.
• You have other related factors that challenge Naegele’s rule. Studies have shown that other factors can influence the length of your pregnancy. These include maternal factors like weight and height, ethnicity, and errors in menstrual and ovulation dates. In these cases, the ultrasound method can be used to adjust your due date.

Knowing when you will finally hold your unborn baby in your arms is extremely important for any expectant mother. Our pregnancy due date calculator will generate your expected due date in seconds.

Plus, our pregnancy tracker also gives you the chance to follow your baby’s development week by week.

It’s our unique way of helping you to be closely connected to your baby, even before its arrival.

• American Pregnancy Association: Pregnancy Due Date Calculator.
• ACOG: Methods for Estimating the Due Date. 
• Lawson GW. (2021). Naegele's rule and the length of pregnancy - A review. Aust N Z J Obstet Gynaecol. 61(2):177-182. 
• Bennett KA, Crane JM, O'shea P, Lacelle J, Hutchens D, Copel JA. (2004). First trimester ultrasound screening is effective in reducing postterm labor induction rates: a randomized controlled trial. Am J Obstet Gynecol.
• Nabil A. et al. (1994). Modified Naegele’s Rule for Determination of the Expected Date of Delivery Irrespective of the Cycle Length. Med. J. Cairo Univ., Vol. 62(1), 39 – 47.