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. Trying to conceive? The ovulation calculator uses the same inputs to predict your fertile window for the current cycle.
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
The standard medical formula is Naegele's rule: estimated due date = first day of your last menstrual period (LMP) + 280 days, or 40 weeks. The rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the due date shifts by the same number of days the cycle differs from 28 — that's why this calculator asks for your average cycle length.
When your cycles are regular and you remember the first day of your last period accurately, Naegele's rule is the same formula obstetricians use everywhere in the world. That said, only about 5% of babies arrive on the exact estimated date — most are born within two weeks before or after. Your dating ultrasound (typically scheduled between weeks 8 and 12) will refine the estimate based on the embryo's actual size and is the most accurate way to confirm your due date.
If your cycles vary by more than a few days month-to-month, an LMP-based due date is less reliable because the calculator assumes ovulation lands at a predictable point. Use the calculator anyway as a starting estimate, then let your dating ultrasound do the precision work. Conditions like PCOS, thyroid imbalance, or recent hormonal contraception can all cause irregular cycles — mention these to your provider.
Pregnancy is measured in gestational weeks counted from the first day of your last menstrual period — not from conception. So if you ovulated about 14 days after your LMP and conceived shortly after, you're already considered "4 weeks pregnant" the day you'd normally expect your next period. This calculator shows your current week-and-day count automatically once you enter your LMP.
If the ultrasound's measurement of the embryo (usually crown-rump length) suggests the pregnancy is significantly older or younger than your LMP indicates, your provider will update the due date based on the scan. ACOG considers the ultrasound dating more accurate than LMP when the difference is more than 5–7 days in the first trimester. The new date is still an estimate — most babies don't arrive on the exact day either way.
Both arrive at the same date when the inputs are accurate — they just start the counting from different points. LMP-based adds 280 days to your last period; conception-based adds 266 days to the day of conception. The 14-day difference is roughly when ovulation (and conception) typically occurs after the start of a period. Use LMP-based when you remember your last period; use conception-based when you've tracked ovulation precisely.
For IVF pregnancies, the most accurate dating uses the embryo transfer date and embryo age, not LMP. Our conception-date calculator includes an IVF mode that handles 3-day and 5-day transfers correctly. Using this LMP calculator for an IVF pregnancy will be off by however much your assumed cycle length differs from the embryo timing.
Most providers schedule a first prenatal visit between 8 and 12 weeks of gestation — which is 4 to 8 weeks after a missed period. If you have a history of miscarriage, a chronic condition like diabetes or hypertension, or you conceived through fertility treatment, your provider may want to see you sooner. Call them once your home test is positive; they'll recommend the right timing.
The standard 40-week figure (280 days from LMP) is an average — actual pregnancy length varies. About 80% of babies are born between 37 and 42 weeks. Babies born before 37 weeks are considered preterm; those born after 42 weeks are post-term and usually delivered by induction. The 38–42 week window is normal and healthy.
Yes — twin pregnancies are dated by the same Naegele's rule, but they almost never go to 40 weeks. Most twin pregnancies deliver around 36–37 weeks, and triplet pregnancies often earlier. Your provider will set a planned delivery date during the third trimester based on the type of twins (identical vs fraternal), shared placenta or not, and how the pregnancy is progressing.
Sources and medical references
Every figure on this page is grounded in published guidance from major medical bodies.