Pregnancy Due Date Calculator — How Your Due Date Is Calculated

Three Ways to Calculate Your Due Date

The pregnancy due date calculator supports three calculation methods. Which you use depends on what information you have:

• Last Menstrual Period (LMP) — most common. Enter the first day of your last period and your cycle length. The calculator applies Naegele's Rule — adding 280 days (40 weeks) to your LMP — adjusted for cycles that differ from 28 days.
• Conception date. If you know when you ovulated or had a positive LH surge, enter that date. The calculator adds 266 days (38 weeks) to find your due date.
• IVF transfer date. For IVF pregnancies, the embryo's age at transfer is precisely known. Enter your transfer date and choose 5-day blastocyst (the most common) or 3-day embryo — the calculator adjusts accordingly.

The Formula: Naegele's Rule Explained

The standard LMP method uses Naegele's Rule, developed in the early 19th century and still used universally:

• Standard due date = LMP date + 280 days (40 weeks)
• Cycle adjustment: if cycle length ≠ 28, add (cycle length − 28) days
• Example: LMP on 1 January, 32-day cycle → due date = 1 January + 280 + 4 = 10 October

The 280 days assumes ovulation on day 14 of a 28-day cycle. The luteal phase (ovulation to next period) is consistently 14 days across cycle lengths — so longer cycles simply mean ovulation happens later, shifting the due date forward by the same number of days.

For IVF: a 5-day blastocyst is 5 days old at transfer, equivalent to conception happening 5 days earlier. Due date = transfer date + 261 days (266 − 5). A 3-day embryo gives transfer date + 263 days.

Gestational Age vs Fetal Age

These two terms are often confused:

• Gestational age is counted from the LMP — the clinical standard used by all healthcare providers and ultrasound measurements. It includes the ~2 weeks before conception actually occurred. When a scan says “you are 8 weeks pregnant,” it means 8 weeks from LMP, not 8 weeks since conception.
• Fetal age (embryonic age) is counted from conception — approximately 2 weeks less than gestational age. This is rarely used clinically.

The calculator displays gestational age — the standard your midwife or OB will use.

Key Pregnancy Milestones by Week

Will My Baby Come on the Due Date?

Fewer than 5% of babies are born on their exact estimated due date. Spontaneous labour follows a roughly normal distribution, with most births occurring in the two weeks surrounding the EDD. A pregnancy is considered “term” between 37 and 42 weeks. After 41–42 weeks, most providers discuss induction to reduce post-term risks.

An early ultrasound (before 13 weeks) is more accurate than LMP calculation for dating, because it directly measures the embryo. If the ultrasound date differs by more than 5–7 days from your LMP calculation, your provider will typically revise the due date to match the scan.

Using the Calculator for Cycle Lengths Other Than 28 Days

If you have a consistently longer or shorter cycle, the standard 28-day formula underestimates or overestimates your due date by exactly the difference. Examples:

• 24-day cycle: Ovulation occurs around day 10, so due date is 4 days earlier than the standard calculation.
• 35-day cycle: Ovulation around day 21, so due date is 7 days later than standard.

The calculator adjusts for this automatically. If you also use the ovulation calculator to confirm your ovulation timing each cycle, you can use the resulting conception date to calculate your due date even more precisely.

Common Questions

My due date changed after an ultrasound — which is correct?

The ultrasound date is generally more accurate in early pregnancy. Crown-to-rump length measurement before 13 weeks is accurate to within ±5–7 days. After 13 weeks, fetal size becomes more variable and ultrasound dating is less reliable. If your early ultrasound differs significantly from your LMP-based date, use the ultrasound date for all planning purposes.

I am not sure of my LMP — what should I do?

If you do not remember your LMP or have irregular cycles, an early dating ultrasound is the most reliable option. Your provider can also estimate gestational age from a transvaginal ultrasound measurement. Until then, use your best estimate for the LMP date — the calculator result gives you a useful starting range even if it gets refined later.

Why does the calculator add 2 weeks before the pregnancy “started”?

Gestational age is counted from the LMP, not conception, because ovulation and fertilisation timing are usually unknown while the period date is reliably remembered. This convention means “week 1” and “week 2” of pregnancy actually precede conception. The embryo itself is about 2 weeks younger than the gestational age reading — a convention that confuses many people but is universally used for consistency across providers and studies.