How accurate is the due date?

Only about 4–5% of babies are born on their exact due date. About 50% are born within 1 week of the EDD; about 90% within 2 weeks. The American College of Obstetricians and Gynecologists considers 37–42 weeks as full-term, with optimal delivery typically between 39 and 41 weeks. An ultrasound in the first trimester provides a more accurate estimate than LMP-based calculation, especially for irregular cycles.

What is Naegele's rule?

Naegele's rule estimates due date by adding 280 days (40 weeks) to the first day of the last menstrual period. It assumes a 28-day cycle with ovulation on day 14. The rule is named after Franz Karl Naegele, a 19th-century German obstetrician who formalized the calculation. Modern calculators adjust the formula for cycles longer or shorter than 28 days.

When do trimesters start?

The first trimester runs weeks 1–13 (some sources use 1–12), the second trimester weeks 14–27, and the third trimester weeks 28–40. Each trimester brings characteristic developmental milestones and pregnancy symptoms. The boundaries are conventions; biological development is continuous, not stepped.

Why is gestational age 2 weeks longer than conception age?

Gestational age (used in pregnancy tracking) is measured from the first day of the last menstrual period — about 2 weeks before actual conception in a typical 28-day cycle. This convention exists because LMP is reliably knowable while conception date is often uncertain. The embryo is biologically about 2 weeks younger than its gestational age suggests.

Should I worry if my ultrasound dating differs from LMP?

Small differences (a few days) are normal. If the first-trimester ultrasound differs from LMP-based calculation by more than 5–7 days, your doctor typically uses the ultrasound date as the official EDD. Ultrasound dating is generally more accurate than LMP in the first trimester. Differences are most commonly due to irregular cycles or imprecisely remembered LMP.

Pregnancy Due Date Calculator

Calculate your estimated due date using Naegele's rule based on the first day of your last menstrual period (LMP). See your current gestational age, trimester, and key pregnancy milestones.

Last updated: May 26, 2026Reviewed by the CalcMountain editorial team

About this calculator

Your estimated due date (EDD) is calculated as 40 weeks (280 days) from the first day of your last menstrual period (LMP). This is the standard convention used by obstetricians, midwives, and pregnancy tracking apps worldwide. The 40-week count is based on Naegele's rule, formulated by a German obstetrician in the early 1800s. It assumes a 28-day cycle with ovulation on day 14 — the calculator adjusts for cycles longer or shorter than 28 days.

A few important realities about due dates: only about 4–5% of babies are actually born on their exact due date. The American College of Obstetricians and Gynecologists (ACOG) considers anything between 37 and 42 weeks as full-term, with optimal delivery typically between 39 and 41 weeks. About 50% of births happen within 1 week of the due date; about 90% happen within 2 weeks before or after. The due date is best understood as the middle of a 4-week likely-delivery window, not a precise prediction.

This calculator computes your due date, current gestational age (how far along you are today), and trimester. It also estimates key milestones — when major fetal development happens, when prenatal testing is typically scheduled, and roughly when to expect "baby is the size of X" updates. For pregnancies conceived through IVF or with known conception date, your fertility clinic's due date estimate (based on embryo transfer date) is more precise than LMP-based calculation.

Formula

Naegele's rule (standard 28-day cycle): Due Date = LMP + 280 days = LMP + 9 months + 7 days Adjustment for cycle length other than 28 days: Adjusted Due Date = LMP + 280 days + (Cycle Length − 28) (A 32-day cycle pushes the due date 4 days later; a 24-day cycle pushes it 4 days earlier.) Current gestational age: Gestational Age (weeks) = (Today − LMP) / 7 Trimesters: First trimester: weeks 1–13 (some texts use 0–12 or 1–12) Second trimester: weeks 14–27 Third trimester: weeks 28–40 Key milestones (gestational age): Week 6–7: cardiac activity detectable by ultrasound Week 10–13: first-trimester screening, NIPT testing Week 15–20: anatomy scan, AFP screening Week 20: anatomy ultrasound (most parents learn baby's sex if not done earlier) Week 24: viability threshold (varies by hospital) Week 28: third trimester begins, glucose tolerance test Week 36: weekly prenatal visits begin Week 37: full-term threshold Week 39–41: optimal delivery window Week 42: post-term, typically induce labor if not delivered Example: LMP January 1, 2026. 28-day cycle. EDD: January 1, 2026 + 280 days = October 8, 2026 For a 32-day cycle: EDD: October 8 + 4 days = October 12, 2026 Today is May 26, 2026. Days since LMP: 145. Gestational age: 145 / 7 ≈ 20.7 weeks (second trimester).

How to use this calculator

Enter the date of the first day of your last menstrual period. This is the most accurate input for due date calculation when using LMP.
Enter your typical cycle length in days. Most women are 28 days; ranges 21–35 days are common. If your cycles are irregular, use your most recent average. For women with very irregular cycles, an early ultrasound provides better dating.
Review the estimated due date and your current gestational age (how far along you are today).
Note the trimester and any upcoming milestone weeks. Use the date to plan prenatal appointments, work leave, and birth preparation.
For pregnancies conceived via IVF, the embryo transfer date provides more accurate dating. The transfer date counts as gestational age 2 weeks 3 days (for day-3 transfers) or 2 weeks 5 days (for day-5 transfers), since the embryo was developing before transfer.
For pregnancies with uncertain or irregular LMP, early ultrasound (first trimester) provides the most accurate due date estimate. Your doctor will likely use the ultrasound date if it differs from LMP-based calculation by more than 5–7 days.
Track key milestones with the calculator: the 12-week mark (often when miscarriage risk drops dramatically and you can share news more widely), 20 weeks (anatomy scan), 28 weeks (third trimester), 37 weeks (full-term).
Remember that the due date is the MIDPOINT of a likely delivery window, not a precise prediction. Don't over-anchor to the specific date.

Worked examples

Standard 28-day cycle, last menstrual period February 1

LMP: February 1, 2026. Cycle: 28 days. EDD: February 1 + 280 days = November 8, 2026. If today is May 26, 2026 (about 114 days after LMP), gestational age = 114 / 7 = 16.3 weeks. Second trimester. Anatomy scan typically scheduled around week 20 (June 21).

Longer cycle adjustment

LMP: March 15, 2026. Cycle: 32 days (4 days longer than average). Standard EDD: March 15 + 280 days = December 20, 2026. Adjusted EDD: December 20 + 4 days = December 24, 2026. The 4-day adjustment matters in early pregnancy when ultrasound dating may be checked against LMP — a mismatch of 5+ days typically prompts the doctor to use the ultrasound date as the official EDD.

IVF pregnancy — known transfer date

IVF transfer: April 10, 2026 (day-5 embryo transfer). For day-5 transfer: gestational age at transfer = 2 weeks 5 days EDD = transfer date + 263 days (a 280-day pregnancy minus the 17 days already counted in the embryo) EDD ≈ December 29, 2026 IVF dating is typically considered more accurate than LMP-based for IVF pregnancies. Fertility clinics provide the official EDD based on transfer date and embryo age.

When to use this calculator

Use this calculator at the start of pregnancy (after a positive home pregnancy test) to estimate your due date and current gestational age. Update as needed if your doctor adjusts the EDD based on an early ultrasound (especially for irregular cycles or uncertain LMP).

It's also useful for planning: maternity leave timing, prenatal appointments, baby shower scheduling, when to install the car seat, when to take time off work, and matching the calendar to typical developmental milestones.

Pair this with the ovulation-calculator (the inverse — when to expect ovulation if planning to conceive), the pregnancy-weight-gain calculator (for tracking healthy weight gain across trimesters), and the BMR/calorie calculators (since calorie needs increase during second and third trimesters).

Important caveats:

1. **Due dates are estimates, not promises.** Only 4–5% of babies are born on the exact due date. About 50% are born within a week of EDD; 90% within 2 weeks. Plan for a 4-week window centered on the EDD.

2. **First-trimester ultrasound usually trumps LMP.** If you can't pinpoint your LMP, or if your cycles were irregular, the doctor's ultrasound dating in the first trimester is more accurate than the calculator. Your "official" EDD may shift slightly based on ultrasound.

3. **Post-term concerns rise after 42 weeks.** Doctors typically schedule induction by 41–42 weeks if labor hasn't started, due to increased complications associated with very long pregnancies. Birth before 37 weeks is considered preterm with its own risks.

4. **Some apps and calculators use slightly different conventions.** Most use Naegele's rule (LMP + 280 days). Some use "conception date + 266 days" (which produces the same EDD for a 28-day cycle but differs for longer/shorter cycles). The calculator above uses the standard LMP+280 convention.

This calculator is a planning tool. For medical decisions, always rely on your obstetrician, midwife, or fertility specialist's assessment.

Common mistakes to avoid

Treating the due date as a guaranteed delivery date. Only ~5% of babies arrive exactly on the EDD. Plan for delivery within 2 weeks before or after; about 1 in 10 babies arrive outside even that window.
Forgetting cycle length adjustment. A 32-day cycle pushes due date roughly 4 days later than the standard 28-day calculation. For irregular cycles, an early ultrasound provides better dating than LMP.
Confusing weeks pregnant with months pregnant. Pregnancy is measured in weeks (gestational age), not months. "9 months pregnant" loosely means weeks 36–40; "third trimester" runs weeks 28–40. The non-uniform mapping is why pregnancy apps use weeks.
Anchoring social and work decisions too tightly to EDD. Maternity leave plans that assume delivery exactly on EDD are fragile — labor 2 weeks early or 1 week late is common. Build flexibility.
Conflating gestational age (from LMP) with "actual age" of the embryo/fetus. Gestational age includes ~2 weeks before conception (the embryo is biologically about 2 weeks younger than its gestational age suggests). This convention is universal but can confuse first-time parents.
Worrying about minor discrepancies between LMP date and ultrasound date. Differences of a few days are normal. The doctor uses the most reliable indicator: first-trimester ultrasound > LMP > later ultrasound.

Frequently Asked Questions

Sources & further reading

Estimated Due Date — clinical guidance — American College of Obstetricians and Gynecologists
Pregnancy — Stages and Care — U.S. Office on Women's Health
Pregnancy Care — CDC overview — U.S. Centers for Disease Control and Prevention

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Pregnancy Due Date Calculator

Calculate your estimated due date using Naegele's rule based on the first day of your last menstrual period (LMP). See your current gestational age, trimester, and key pregnancy milestones.

Inputs

LMP Month (1-12)

LMP Day (1-31)

LMP Year

Cycle Length (days)

Average menstrual cycle length

Results

Due Date

October 8, 2026

Current Week

36w 5d

Trimester

Third Trimester

Days Remaining

Pregnancy Milestones

Week	Milestone	Date
6	Heartbeat detectable	February 12, 2026
12	End of first trimester	March 26, 2026
20	Anatomy scan / halfway	May 21, 2026
24	Viability milestone	June 18, 2026
27	Third trimester begins	July 9, 2026
37	Early term	September 17, 2026
40	Due date	October 8, 2026