How this due date calculator works
This tool focuses on your estimated due date (EDD) from four starting points: last menstrual period (LMP) with optional average cycle length (Naegele-style rule with adjustment), conception date (+266 days), ultrasound date plus gestational age from LMP (works back to LMP and EDD), and IVF embryo transfer with Day 3 or Day 5 embryo (+263 or +261 days to EDD). For a full week-by-week schedule and milestones from any mode, use the pregnancy calculator.
What is a due date?
The due date — also called estimated date of confinement (EDC) or estimated date of delivery (EDD) — is an estimate of when birth may occur. It often corresponds to 40 weeks gestational age from the first day of the LMP. The word "estimated" matters: only a small fraction of babies arrive on the exact day; roughly 60% are within a week and 90% within two weeks. Think of the EDD as the middle of a typical window around weeks 38–42, not a deadline.
Methods for estimating due date
Last menstrual period (LMP)
The usual clinical shortcut is Naegele's rule: add 280 days (40 weeks) to the first day of the LMP, which is equivalent to LMP + 9 months + 7 days for a 28-day cycle. Example: LMP February 7, 2026 → about November 14, 2026. If your average cycle is not 28 days, ovulation (and thus conception) tends to shift about one day per day your cycle differs from 28 — this calculator moves the EDD by that amount (cycle length is clamped to a reasonable range for stability).
Conception date
From a known or estimated conception (ovulation) date, add 266 days (38 weeks) to approximate the EDD. That reflects the ~2-week offset between LMP and fertilization in a typical cycle. Best when you track ovulation; less reliable if cycles are irregular and the conception day is uncertain.
Ultrasound dating
Enter the scan date and the gestational age the sonographer reported (from LMP). The calculator derives implied LMP and EDD. First-trimester dating (often ~8–13 weeks, crown–rump length) is usually the most accurate window to adjust the EDD; if ultrasound and LMP disagree by more than about a week, many clinicians favor first-trimester ultrasound. Later scans are less reliable for changing the due date because fetal size varies more.
IVF transfer date
IVF gives a fixed reference: Day 5 (blastocyst) transfer → EDD ≈ transfer + 261 days; Day 3 transfer → EDD ≈ transfer + 263 days (266 days from fertilization minus embryo age at transfer). Your clinic's protocol and documentation remain authoritative.
Understanding term pregnancy
Labels have evolved as outcomes were studied within 37–42 weeks. Commonly: preterm before 37 weeks; early term ~37–38 weeks (often monitored more than "classic" full term); full term often emphasized around 39–40 weeks for best average outcomes; late term ~41–42 weeks with closer surveillance; post-term beyond 42 weeks — many practices discuss induction by then because placental function can wane. Your OB or midwife applies current guidance to your situation.
Gestational age — how pregnancy weeks are counted
Gestational age counts from the first day of the LMP, not from conception. At fertilization you are already ~2 weeks "pregnant" by that clock. Forty weeks from LMP is about 38 weeks from conception. LMP is used because it is usually a known, objective anchor; conception time is rarely known exactly. When someone says "12 weeks pregnant," they almost always mean 12 weeks gestational from LMP.
Factors that can affect actual delivery date
- Maternal age, prior births, and health conditions (e.g. diabetes, hypertension).
- Multiples (twins and more) usually deliver earlier than singleton due dates suggest.
- Genetic and unknown variation — even large studies leave residual scatter in natural pregnancy length.
Use this page for education and planning; medical decisions belong with your prenatal team.
Typical prenatal timeline (overview)
- ~8–10 weeks: First visit, history, labs, dating discussion, sometimes early ultrasound.
- ~10–13 weeks: First-trimester screening options (e.g. NT + blood, or cfDNA/NIPT per protocol).
- ~18–22 weeks: Anatomy ultrasound.
- ~24–28 weeks: Glucose screening for gestational diabetes.
- ~35–37 weeks: Group B strep screening; third-trimester planning.
- 36 weeks onward: More frequent visits; added testing if late or post-term.