About the Due Date Calculator
The estimated due date (EDD) anchors all of prenatal care: which week you're in decides when screening tests happen, how growth is judged, and how “early” or “late” is defined at the other end. Obstetrics counts pregnancy as 40 weeks from the last menstrual period — which, counterintuitively, means the clock starts about two weeks before conception.
Pick your best-known starting point. Most people know their last period, so LMP is the default — with a cycle-length adjustment most simple calculators skip. If you know the conception date, or you conceived through IVF (where the embryo's age is known to the day), those are more precise anchors. The result includes how many weeks pregnant you are today and the days remaining.
Tracking a healthy pregnancy week by week? Pair this with the Pregnancy Weight Gain Calculator.
The Four Dating Methods
All four methods aim at the same target — 266 days (38 weeks) from conception — they just start counting from different known events:
LMP: due date = last period start + 280 days + (cycle length − 28) Conception: due date = conception date + 266 days IVF day-5: due date = transfer date + 261 days IVF day-3: due date = transfer date + 263 days
| Starting point | Days added | Example (July 1, 2026) | Due date |
|---|---|---|---|
| Last menstrual period (28-day cycle) | 280 | July 1, 2026 | April 7, 2027 |
| Conception date | 266 | July 1, 2026 | March 24, 2027 |
| IVF day-5 embryo transfer | 261 | July 1, 2026 | March 19, 2027 |
| IVF day-3 embryo transfer | 263 | July 1, 2026 | March 21, 2027 |
The old textbook shortcut — Naegele's rule: LMP minus 3 months plus 7 days — approximates the same 280 days and can drift a day depending on month lengths; this calculator counts actual days, per ACOG's definition.
Trimesters at a Glance
Pregnancy is counted in weeks of gestational age (from the LMP-equivalent start). The trimester boundaries this calculator uses:
| Trimester | Weeks | Highlights |
|---|---|---|
| First | 1–13 | Dating ultrasound, first screenings; organs form |
| Second | 14–27 | Anatomy scan (~20 wk); movement felt (~18–22 wk) |
| Third | 28–40+ | Growth checks; full term begins at 37 weeks |
“Full term” spans 39 weeks 0 days to 40 weeks 6 days in ACOG's definitions, with 37–38 weeks called early term and 41+ late term — the due date sits in the middle of a normal range, not at its edge.
How Accurate Are Due Dates?
An EDD is a statistical midpoint, not an appointment: only about 1 in 20 babies arrives on the due date itself, and most arrive within roughly two weeks either side. LMP dating inherits every uncertainty of cycle recall and ovulation timing, which is why ACOG considers a first-trimester ultrasound the most accurate dating method — if an early scan disagrees with your LMP date meaningfully, your provider will re-date the pregnancy to the scan.
Practical reading: treat the due date as the center of a due month. Plan leave, travel, and logistics around weeks 37–42 rather than one calendar day — and let your prenatal care team make the official call, since they'll combine the calendar with ultrasound and exam findings this calculator can't see.