Due Date Calculator Online

How to Calculate Your Pregnancy Due Date

1. Pick a Dating Method: Choose Last Menstrual Period (LMP) for the standard Naegele's rule calculation, Conception Date if you tracked ovulation, Ultrasound Date (with gestational age in weeks + days from the scan), or IVF Transfer Date with embryo stage (Day 3, Day 5/blastocyst, or Day 6).
2. Enter Your Date and Cycle Length: Type the relevant date and set your Average Cycle Length (default 28 days; adjust to your actual cycle — Naegele's rule presumes 28 days with ovulation on day 14, which only about 13 percent of cycles match).
3. Review the Results: The calculator returns your Estimated Due Date (EDD), your Current Gestational Age in weeks + days, the Trimester breakdown, and key milestones (NT scan, anatomy scan, glucose test, GBS swab) with their target date windows.
4. Save or Share: Copy the EDD, screenshot the milestone timeline, or note the date for prenatal appointments. Calculations run entirely in your browser — no account, no email, no health-record storage on our side.

Why Use a Due Date Calculator?

A due date calculator translates a single date — your LMP, conception, ultrasound, or IVF transfer — into the 280-day pregnancy timeline obstetricians use, so you can plan appointments, maternity leave, and birth prep against the same calendar your provider uses. Only about 5 percent of babies arrive on the calculated EDD; the rest fall in a roughly two-week window on either side, which is why providers track the EDD as a planning target rather than a deadline. Typical reasons to run the numbers yourself:

• Confirm the date your provider gave you — first-trimester ultrasound (crown-rump length, CRL) is the gold standard; ACOG redates the EDD if the scan differs from LMP by more than 5 days at 8 6/7 weeks or earlier, or more than 7 days through 13 6/7 weeks.
• Plan prenatal appointments — NT scan + first-trimester screen at 10-13 weeks, anatomy scan at 18-22 weeks, glucose challenge at 24-28 weeks, GBS swab at 36 0/7-37 6/7 weeks (per ACOG Committee Opinion 797).
• Schedule maternity leave and FMLA paperwork — US FMLA eligibility uses an expected date; an accurate EDD lets HR start the 12-week clock at the right time.
• Track IVF pregnancies precisely — a Day 5 blastocyst transfer plus 261 days gives the EDD; Day 3 embryo plus 263 days; the underlying 266-day post-fertilisation interval is well-defined, so IVF EDDs are usually tighter than LMP-derived ones.
• Adjust expectations for twins — uncomplicated dichorionic twins are typically delivered at 38 0/7-38 6/7 weeks, monochorionic-diamniotic at 36 0/7-37 6/7, and monochorionic-monoamniotic at 32-34 weeks by planned cesarean; the EDD stays the same, but planning shifts earlier.
• Know when "post-term" applies — ACOG defines late-term as 41 0/7-41 6/7 weeks and post-term as 42 0/7 weeks and beyond; induction can be considered after 41 weeks and is recommended by 42 6/7 weeks.

For a working-day count between any two dates (handy for FMLA windows or counting back from the EDD), pair this with Convert Date to Timestamp.

Due Date Methods — Which Is Most Accurate

Method	Formula	Typical accuracy	When to use
LMP (Naegele's rule)	First day of LMP + 280 days (or LMP + 7 days, then minus 3 months)	±1-2 weeks; assumes 28-day cycle, ovulation day 14	First trimester before any ultrasound, regular 28-day cycles
Conception date	Conception + 266 days	±3-5 days if ovulation was confirmed (OPK, BBT, ultrasound follicle scan)	Tracked ovulation cycles, known conception window
First-trimester CRL ultrasound	Gestational age from CRL measurement (mm)	±5-7 days through ~13 6/7 weeks; ACOG's preferred dating method	All pregnancies once an early ultrasound is available
IVF Day 5 transfer	Transfer date + 261 days	±1 day (precise embryology)	Fresh or frozen blastocyst transfer
IVF Day 3 transfer	Transfer date + 263 days	±1 day	Cleavage-stage embryo transfer
Second-trimester ultrasound (14-22 wk)	Biparietal diameter, head circumference, femur length	±7-10 days	Late-booking pregnancies, unsure LMP
Third-trimester ultrasound (28+ wk)	Composite biometry	±2-3 weeks (least reliable)	Unbooked pregnancies — confirm only if discrepancy exceeds 21 days

ACOG's redating thresholds: a discrepancy of more than 5 days at ≤8 6/7 weeks, more than 7 days at 9 0/7-13 6/7 weeks, more than 10-14 days at 14 0/7-21 6/7 weeks, more than 14 days at 22 0/7-27 6/7 weeks, and more than 21 days at 28+ weeks all shift the EDD to the ultrasound estimate.

Pregnancy Milestones by Week

Gestational age	Milestone	Why it matters
5-6 weeks	Gestational sac visible on transvaginal ultrasound	First confirmation of intrauterine pregnancy
6-7 weeks	Fetal cardiac activity detectable	Strong predictor of ongoing pregnancy
8 weeks	First prenatal visit (most US practices)	Confirm dating, bloodwork, prenatal vitamin review
10-13 weeks	NT scan + first-trimester combined screen (NIPT cell-free DNA from 9-10 weeks)	Screens for trisomy 21, 18, 13; some labs report fetal sex from 10 weeks
11-13 6/7 weeks	CVS available if indicated	Diagnostic karyotype option
15-22 weeks	Quad/serum screen, amniocentesis if indicated	Neural-tube and chromosomal screening alternative to NIPT
18-22 weeks	Anatomy scan (Level II ultrasound)	Detailed fetal structural survey; fetal sex usually visible
24-28 weeks	Glucose challenge test (50 g) — earlier if risk factors	Gestational diabetes screening
27-36 weeks	Tdap booster (ACIP recommends 27-36 weeks every pregnancy)	Pertussis antibodies for newborn protection
32-36 weeks	RSV maternal vaccine (Abrysvo, weeks 32-36 in RSV season)	Reduces infant RSV hospitalisation
36 0/7-37 6/7 weeks	Group B Strep (GBS) swab	Determines intrapartum antibiotic prophylaxis
37 0/7-38 6/7 weeks	Early term — delivery elective only if medically indicated	Avoid non-medically-indicated delivery before 39 weeks
39 0/7-40 6/7 weeks	Full term — routine surveillance	Most spontaneous labours occur in this window
41 0/7-41 6/7 weeks	Late term — increased antenatal surveillance; induction may be offered	Stillbirth risk begins rising
42 0/7+ weeks	Post-term — induction recommended by 42 6/7 weeks	Macrosomia, oligohydramnios, and stillbirth risks climb

Vaccine windows reflect current CDC/ACIP recommendations as of the 2025-2026 season; check with your provider for the latest schedule.

Frequently Asked Questions

How accurate is Naegele's rule, and why is it sometimes off?

Naegele's rule (LMP + 280 days, or LMP + 7 days then subtract 3 months) is a population average, not a personalised estimate. It assumes a 28-day menstrual cycle with ovulation on day 14 — only about 13 percent of cycles actually fit that profile. For a 35-day cycle the formula tends to date the pregnancy roughly a week earlier than reality; for a 21-day cycle, roughly a week later. Even in textbook 28-day cycles, studies show Naegele's rule places the EDD about 2-4 days too early on average. Only about 5 percent of babies arrive exactly on the LMP-calculated date.

Why is first-trimester ultrasound the gold standard for dating?

Between 7 and 13 6/7 weeks the embryo's crown-rump length (CRL) grows on a tight, well-characterised curve that does not vary much between pregnancies. A single CRL measurement in that window dates the pregnancy to within roughly ±5-7 days. After ~14 weeks (CRL exceeds 84 mm), the curve flattens and other biometric parameters (biparietal diameter, head circumference, femur length) take over — those are less precise (±7-21 days, worsening with gestational age). ACOG Committee Opinion 700 codifies this: an early CRL scan overrides the LMP-derived EDD whenever the discrepancy passes the redating threshold for that gestational age.

How are trimesters defined — calendar weeks or medical weeks?

Most US practices use the obstetric convention: first trimester runs 0-13 6/7 weeks (LMP), second 14 0/7-27 6/7 weeks, third 28 0/7 weeks to delivery. Some sources draw the lines at 12 and 28, or use months (1-3, 4-6, 7-9). The differences are minor in practice, but if you are reading two sources that disagree on whether you are "in the second trimester" by a few days, it is almost always a definition mismatch rather than a dating error.

When does pregnancy actually "start" — and why is week 1 before conception?

Standard obstetric dating starts the clock at the first day of the LMP, not at conception. That convention exists because the LMP is usually the only date a pregnant person can pinpoint, whereas the conception day is rarely known precisely. The trade-off is that the first two weeks of "pregnancy" cover the menstrual and proliferative phases — conception itself happens around the end of week 2. A 5-day blastocyst transfer in IVF is already considered "2 weeks 5 days pregnant" on the day of transfer.

How do IVF transfers change the calculation?

IVF EDDs work backward from a known fertilisation date rather than an estimated ovulation. The 266-day post-fertilisation interval is fixed; the embryo's age at transfer adjusts the formula. A Day 5 blastocyst transfer becomes transfer date + 261 days (266 - 5). A Day 3 cleavage embryo becomes transfer date + 263 days. A Day 6 transfer becomes transfer date + 260 days. Frozen embryo transfers (FET) follow the same rules — the freeze-thaw cycle pauses development, so it's the embryo age at transfer that matters, not the original retrieval date. Because IVF eliminates the cycle-length and ovulation-timing variability, these EDDs are usually closer to the actual delivery date than LMP-derived ones.

My due date passed. What happens now?

ACOG defines late term as 41 0/7-41 6/7 weeks and post-term as 42 0/7 weeks and later. Antenatal surveillance (non-stress tests, biophysical profile, amniotic-fluid assessment) is generally added at 41 weeks. Induction of labour can be considered at 41 0/7-42 0/7 weeks — the 2018 ARRIVE trial showed elective induction at 39 weeks did not increase adverse outcomes, and the 2019 Cochrane review on induction at or beyond term found a small reduction in perinatal death. ACOG recommends induction by 42 6/7 weeks because perinatal morbidity and stillbirth risk climb meaningfully past 42 weeks.

Are twins really due earlier?

The calculated EDD is the same for singletons and twins (and triplets) — it is derived from LMP, conception, or transfer date, none of which depend on the number of fetuses. What changes is the planned delivery window. ACOG and SMFM recommend delivery at 38 0/7-38 6/7 weeks for uncomplicated dichorionic-diamniotic (DCDA) twins, 36 0/7-37 6/7 weeks for monochorionic-diamniotic (MCDA) twins, and 32 0/7-34 0/7 weeks by planned cesarean for monochorionic-monoamniotic (MCMA) twins. Spontaneous twin delivery averages around 35-36 weeks even in DCDA pregnancies — earlier than singletons, but usually planned around the recommended targets.

Can my due date change during pregnancy?

Yes — but usually only once, and only in the first trimester. If a first-trimester CRL ultrasound shifts the EDD past the ACOG threshold (more than 5 days at ≤8 6/7 weeks; more than 7 days at 9 0/7-13 6/7 weeks), your provider will update it and that becomes the working EDD for the rest of the pregnancy. Second-trimester scans only redate if they differ by more than 10-14 days; third-trimester scans only if more than 21 days. Once an EDD is established by an early ultrasound, late-pregnancy fetal-size estimates ("the baby is measuring big/small") do not move the EDD — they describe growth, not dating.

Are the calculations or my dates stored anywhere?

No. The calculator runs entirely in your browser — your LMP, conception, or transfer date never leaves your device. There is no sign-up, no email capture, and nothing is written to a server or shared with third parties. Close the tab and the data is gone; refresh and you start fresh. For anything beyond a planning estimate (especially decisions about induction, scheduled cesarean, or work leave), confirm the EDD with your prenatal provider — they have access to your ultrasound measurements, which override any LMP-based estimate.