Calculator · Last updated May 6, 2026
Due date calculator
Enter the first day of your last menstrual period and your average cycle length. We'll estimate your due date, current week, and trimester — using the same formula your doctor uses.
Written and reviewed by the babybumpkit editorial team, drawing on guidance from ACOG, the Mayo Clinic, and the NHS.
How this calculator works
The standard medical formula for estimating a due date is Naegele's rule: due date = first day of your last menstrual period (LMP) + 280 days. That's 40 weeks, the same number you'll hear from your doctor or see in any prenatal app.
Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter than 28 days, ovulation shifts — and so does the due date. This calculator asks for your average cycle length and adjusts the result accordingly. The formula it uses is published in the ACOG committee opinion on dating.
All math runs in your browser — your dates are never sent to our servers. We use UTC throughout so a late-night entry won't shift the result by a day.
Why cycle length matters
For someone with a textbook 28-day cycle, Naegele's rule works perfectly. For everyone else — which is most people — the standard formula can be off by a few days because ovulation happens later in longer cycles and earlier in shorter ones.
The general adjustment: every day your cycle is longer than 28 days pushes your due date one day later, and every day it's shorter pulls it one day earlier. So a 31-day cycle gives a due date three days later than the textbook calculation; a 26-day cycle gives one two days earlier.
If your cycles vary by more than a few days month-to-month — common with PCOS, thyroid issues, or after stopping hormonal contraception — Naegele's rule is less reliable. In that case, treat the result as a starting estimate and let your dating ultrasound do the precision work.
LMP vs. conception date — what's the difference?
Pregnancy is measured from the first day of your last period (LMP), not from conception. That's a clinical convention that exists because most people remember their last period more reliably than the day they ovulated. The catch: LMP-based gestational age is about two weeks ahead of fetal age (counted from conception).
That's why a pregnancy is described as 40 weeks long when fetal development takes 38 weeks. Both numbers describe the same pregnancy; they just start the clock at different points.
If you tracked ovulation with LH strips or a fertility monitor, the conception-date calculator gives a more precise result. Use this LMP version when you don't know your exact ovulation day — which covers most people.
How accurate is an LMP-based due date?
When your cycles are regular and your LMP date is accurate, Naegele's rule is the same baseline obstetricians use worldwide. That said, only about 5% of babies are born on the exact estimated date. Most arrive within two weeks before or after.
Your dating ultrasound — usually scheduled between 8 and 12 weeks — measures the embryo directly and is the most accurate way to confirm or refine the due date. ACOG guidance says the ultrasound estimate replaces the LMP-based estimate when they differ by more than 5–7 days in the first trimester.
If you don't remember the exact day of your last period, even an approximate date plus an early ultrasound will get you to a reliable due date.
When to schedule your first prenatal visit
Most providers schedule a first prenatal visit between weeks 8 and 12 of gestation — which is 4 to 8 weeks after a missed period. That timing lets the dating ultrasound be informative while leaving room to start prenatal vitamins, screening, and other early-pregnancy care.
If you have a history of pregnancy loss, a chronic condition like diabetes or hypertension, or you conceived through fertility treatment, your provider may want to see you sooner. Don't wait for symptoms — early care matters even when everything feels fine.
Frequently asked questions
Sources and medical references
Every figure on this page is grounded in published guidance from major medical bodies.
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