Pregnancy Due Date: How It's Calculated

How pregnancy due date works in practice

Estimated due date (EDD) uses Naegele's rule: LMP + 1 year − 3 months + 7 days. Ultrasound dating in the first trimester can adjust EDD if cycle length is irregular or LMP is uncertain. Gestational age is measured from LMP, not conception.

The goal is not to memorize every term — it is to know which inputs matter and what outcome you are aiming for.

So what: When you can explain this in your own words, you are far less likely to accept a bad quote, fee, or assumption.

A real scenario worth running

LMP = March 8, 2026; regular 28-day cycles. Step by step: Add 1 year: March 8, 2027 → Subtract 3 months: December 8, 2026 → Add 7 days: December 15, 2026 due date → Conception typically ~2 weeks after LMP → ~March 22, 2026. Bottom line: EDD December 15, 2026. Ultrasound at 8–12 weeks confirms or adjusts by a few days.

So what: Plug your own numbers into the same logic before you decide.

How due dates are calculated

Your pregnancy due date (estimated delivery date, EDD) is typically 40 weeks (280 days) from the first day of your last menstrual period (LMP). Conception usually occurs about 2 weeks after LMP — so fetal age is ~2 weeks less than gestational age.

Naegele's rule: LMP + 1 year − 3 months + 7 days

Only about 5% of babies arrive on the exact due date. Normal delivery falls between 37 and 42 weeks.

So what: Run your own inputs before you commit — small changes in assumptions can shift the outcome sharply.

Worked example

LMP = March 8, 2026 (regular 28-day cycle):

Estimated conception: ~March 22, 2026.

So what: Run your own inputs before you commit — small changes in assumptions can shift the outcome sharply.

Ultrasound dating

Early first-trimester ultrasound (8–12 weeks) measures crown-rump length for precise dating. Your provider may adjust EDD if ultrasound differs from LMP by more than 5–7 days.

So what: Run your own inputs before you commit — small changes in assumptions can shift the outcome sharply.

Gestational age terms

ACOG recommends elective delivery not before 39 weeks without medical indication.

So what: Run your own inputs before you commit — small changes in assumptions can shift the outcome sharply.

Irregular cycles

If your cycle is 35 days instead of 28, LMP-based dating overestimates gestational age. Ovulation occurs later — ultrasound or ovulation tracking improves accuracy.

Adjusted ovulation ≈ cycle length − 14 days
35-day cycle → ovulation ~day 21

So what: Run your own inputs before you commit — small changes in assumptions can shift the outcome sharply.

What due date doesn't mean

So what: Run your own inputs before you commit — small changes in assumptions can shift the outcome sharply.

Planning around your due date

Build flexibility into:

• Work leave — start disability/FMLA planning by week 32–34
• Childcare — waitlists may need months of lead time
• Hospital bag — packed by week 36
• Support network — due date ± 2 weeks

So what: Run your own inputs before you commit — small changes in assumptions can shift the outcome sharply.

IVF and known conception

IVF pregnancies use embryo transfer date or conception date instead of LMP:

EDD from 5-day embryo transfer = transfer date + 261 days
EDD from 3-day embryo transfer = transfer date + 263 days

Your fertility clinic provides the precise calculation.

So what: Run your own inputs before you commit — small changes in assumptions can shift the outcome sharply.

Common mistakes

1. Standard pregnancy is 40 weeks from LMP (38 weeks from conception) — this quietly costs you over time.
2. First-trimester ultrasound is most accurate for dating — this quietly costs you over time.
3. Full term = 37–42 weeks; preterm is before 37 weeks — this quietly costs you over time.
4. Irregular cycles make LMP-based dates less reliable — this quietly costs you over time.
5. Due date is an estimate — plan for a 2–3 week window..

What to do next

Use our Due Date Calculator to model your situation — change one input at a time to see what moves the result most.