What if my cycle is irregular?

If your cycles are irregular, the LMP method may be less accurate. An early ultrasound (6-9 weeks) is the best way to establish a reliable due date. Your doctor may adjust your due date based on ultrasound measurements, particularly crown-rump length (CRL).

Can my due date change?

Yes. Your healthcare provider may adjust your due date based on early ultrasound measurements, especially if there's a discrepancy of more than 7 days between the LMP-calculated date and the ultrasound-based date. First trimester ultrasounds are the most accurate for dating.

What is the difference between gestational age and fetal age?

Gestational age is counted from the first day of your last menstrual period and is about 2 weeks longer than fetal age. Fetal age (also called embryonic age) is counted from the actual date of conception. When people say pregnancy is 40 weeks, they're using gestational age.

How does the IVF due date calculation differ?

IVF due date calculations are based on the embryo transfer date. For a Day 3 embryo transfer, 263 days are added. For a Day 5 transfer, 261 days are added. These calculations account for the known age of the embryo at the time of transfer, making them very precise.

Why do I need to know my cycle length?

Cycle length affects when you ovulate. A standard 28-day cycle assumes ovulation on day 14. If your cycle is longer (say 35 days), you likely ovulated later, pushing your due date out. The calculator adjusts by adding the difference between your cycle and 28 days.

When should I see a doctor to confirm my due date?

Schedule your first prenatal appointment as soon as you get a positive pregnancy test, ideally between weeks 6-8. An early ultrasound at 8-12 weeks is the gold standard for confirming your due date. The sooner you have a dating ultrasound, the more accurate it is.

Medically reviewed by Samantha L. Fox, RN, BSN, MSN

Nursing Informaticist · United States Navy Nurse Corps · Last reviewed April 30, 2026

📅 Free Due Date Calculator

Pregnancy Due Date Calculator

Find your estimated due date using your last period, conception date, IVF transfer, or ultrasound measurements.

First day of your last period

Average cycle length (days)

How the Due Date Calculator Works

Our pregnancy due date calculator uses four medically recognized methods to estimate when your baby will arrive. Each method applies standard obstetric formulas used by healthcare professionals worldwide, giving you the most accurate estimate possible based on the information you provide.

Last Menstrual Period (LMP) Method — Naegele's Rule

The most commonly used method, Naegele's Rule, calculates your due date by adding 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle with ovulation occurring on day 14. If your cycle is shorter or longer than 28 days, the calculator adjusts by adding or subtracting the difference.

Conception Date Method

If you know the date of conception, the calculator adds 266 days (38 weeks). This is 14 days less than the LMP method because it starts from the actual moment of fertilization rather than the beginning of the menstrual cycle.

IVF Transfer Date Method

For IVF pregnancies, the due date is calculated from the embryo transfer date. A Day 3 embryo transfer adds 263 days, while a Day 5 (blastocyst) transfer adds 261 days. These calculations are the most precise of all methods because the exact age of the embryo is known.

Ultrasound-Based Method

When you have an ultrasound, the technician measures the baby to determine gestational age. Our calculator takes the ultrasound date and the gestational age measured, then projects forward to the 40-week mark. First-trimester ultrasounds are considered the gold standard for dating.

Remember that an estimated due date is just that — an estimate. Only about 5% of babies arrive on their exact due date, but approximately 80% arrive within two weeks of it.

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📚Clinical Sources & References

The calculations and guidance on this page are based on current clinical standards and peer-reviewed research. Reviewed by Samantha L. Fox, RN, BSN, MSN — Emergency Department nurse and US Navy Nurse Corps officer.

[1]ACOG Practice Bulletin No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150-e154.
[2]Naegele FC. Lehrbuch der Geburtshülfe. 1812. (Naegele's rule: add 280 days to LMP for due date estimation.)
[3]Butt K, Lim K; Society of Obstetricians and Gynaecologists of Canada. Determination of gestational age by ultrasound. J Obstet Gynaecol Can. 2014;36(2):171-183.
[4]Hoffman CS, Messer LC, Mendola P, Savitz DA, Herring AH, Hartmann KE. Comparison of gestational age at birth based on last menstrual period and ultrasound during the first trimester. Paediatr Perinat Epidemiol. 2008;22(6):587-596.

Frequently Asked Questions

The most common method adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). If your cycle is longer or shorter than 28 days, the calculation is adjusted accordingly. This is called Naegele's Rule and has been used for over 200 years.

Due date calculators give an estimated due date, not an exact date. Only about 5% of babies are born on their exact due date. Most babies arrive within two weeks before or after the estimated date. Ultrasound-based calculations in the first trimester tend to be most accurate, typically within 5-7 days.