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Free Pregnancy Tool

Due Date Calculator

Estimate when your baby will arrive using one of five clinically-used calculation methods. Choose the method that best matches your situation for the most accurate result.

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Understanding the Methods

How Each Calculation Works

Naegele's Rule (Last Period)

The most commonly used formula in obstetrics, Naegele's Rule estimates your due date by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). This method assumes a regular 28-day cycle with ovulation occurring on day 14. While simple and widely used, it works best for women with consistent, predictable cycles.

Formula: LMP + 280 days = estimated due date

Parikh's Formula (Irregular Cycles)

For women whose menstrual cycles are longer or shorter than 28 days, Parikh's Formula adjusts the standard calculation to account for your actual cycle length. This method is particularly useful if you have consistently irregular cycles, as the standard Naegele's Rule could be off by several days.

Formula: LMP + 280 days + (cycle length − 28 days) = estimated due date

Mittendorf-Williams Rule (First Pregnancy)

Research by Mittendorf and Williams found that first-time mothers tend to have slightly longer pregnancies. Their formula adds 288 days for first pregnancies (about 8 days longer than Naegele's) and 283 days for women who have given birth before. This method accounts for the physiological differences between first and subsequent pregnancies.

Formula: LMP + 288 days (first baby) or LMP + 283 days (not first)

Conception Date Method

If you know your date of conception or ovulation (through ovulation testing, temperature charting, or fertility monitoring), this method provides a straightforward calculation. It adds 266 days (38 weeks) from the date of conception, bypassing the need to estimate ovulation from your last period.

Formula: Conception date + 266 days = estimated due date

IVF Embryo Transfer Method

For pregnancies conceived through IVF, the transfer date is precisely known, allowing for accurate dating. The calculation differs based on the embryo stage at transfer: Day 3 embryos require adding 263 days, while Day 5 blastocysts require adding 261 days (as they are 2 days further in development). Learn more about our fertility scan services.

Formula: Transfer date + 263 days (Day 3) or + 261 days (Day 5)

Confirm Your Due Date with a Dating Scan

While calculators provide a helpful estimate, an ultrasound dating scan is the gold standard for determining your due date. Our experienced sonographers can confirm your dates from as early as 6.5 weeks.

Book an Early Pregnancy ScanContact Us

Common Questions

Frequently Asked Questions

Due date calculators provide an estimate based on averages. Only about 5% of babies arrive on their exact due date. Most are born within 1–2 weeks either side. An early pregnancy ultrasound scan is the most accurate way to confirm your due date, especially when performed between 6.5 and 11 weeks.
Naegele's Rule is the most widely used method. It calculates your due date by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). It assumes a regular 28-day cycle and ovulation on day 14. While useful as a baseline, it may be less accurate for women with irregular cycles.
If your cycle isn't 28 days, the standard Naegele's Rule may not be as accurate. Parikh's Formula adjusts for your actual average cycle length, giving a better estimate. Select the "Irregular Cycle" method above and enter your usual cycle length. A dating scan can also help confirm your dates.
IVF due dates are calculated from your embryo transfer date rather than a last period date. For a Day 3 embryo, add 263 days to your transfer date. For a Day 5 blastocyst, add 261 days. Select the "IVF Transfer" method above to use this calculation. Your fertility clinic will also confirm dating at your early scans.
Gestational age is measured from the first day of your last period and is the standard used in pregnancy (e.g., "you are 12 weeks pregnant"). Fetal age (also called embryonic age) is measured from conception, roughly 2 weeks less. Our calculator displays gestational age, which is what your healthcare provider will use.
A dating scan is most accurate when performed between 6.5 and 13 weeks of pregnancy. At Ultrasound Ireland, we offer early pregnancy scans from 6.5 weeks and later dating scans up to 13.6 weeks. These scans measure your baby to confirm or adjust your due date.
Your due date may be adjusted if the ultrasound measurements show your baby is measuring larger or smaller than expected for the dates calculated from your last period. Early scans are most reliable for dating because all embryos grow at a similar rate in early pregnancy. This is especially common in women with irregular cycles.
The Mittendorf-Williams rule adjusts the expected due date based on whether this is your first pregnancy. Research showed that first-time mothers tend to carry about 5 days longer than the standard 280 days. The formula adds 288 days for first pregnancies and 283 days for subsequent pregnancies.
Yes — select the "Conception Date" method. If you tracked ovulation using tests, temperature charting, or fertility monitoring, enter that date. The calculator adds 266 days (38 weeks) from conception, which is equivalent to 40 weeks from your last period.
While this calculator gives a good estimate, it's important to confirm your due date with an ultrasound scan and your healthcare provider. An early dating scan is the gold standard for due date accuracy. You can contact us to book a scan at any of our three Dublin locations.

Medical Disclaimer

This due date calculator provides an estimate only and should not be used as a substitute for professional medical advice. Every pregnancy is unique, and your actual delivery date may differ from the estimated date. We recommend confirming your due date with an ultrasound dating scan and consulting your healthcare provider for personalised guidance.