PonyatnoHow it's calculated
From an ultrasound: first day of last period = scan date − (weeks × 7 + days). Gestational age on a scan report is already counted from the first day of the last period, so it simply needs winding back.
Ultrasound accuracy depends on when the scan was done — per ACOG Committee Opinion 700, by completed weeks: up to 8 weeks inclusive ±5 days, up to 13 ±7, up to 15 ±7, up to 21 ±10, up to 27 ±14, later ±21 days.
From a due date: first day = due date − 280 days (Naegele's rule in reverse). No range is added: you get exactly the date your pregnancy was dated from.
From conception: first day = conception − (cycle length − 14), ±3 days. It assumes ovulation happens 14 days before the next period.
From the expected period: first day = expected period date − cycle length, ±3 days.
What to know
Why the date matters — and what is more accurate
Gestational age and the due date are counted from the first day of the last period. Forgetting it is very common and not a problem: your pregnancy will be dated by an early ultrasound anyway. Measuring the embryo in the first trimester is the most accurate method, and when it disagrees with the period date, clinicians usually go with the scan.
When the estimate is off
The conception and expected-period methods assume ovulation 14 days before a period. With irregular cycles, after stopping the pill or while breastfeeding, ovulation can come earlier or later and shift the date. Light bleeding in early pregnancy is sometimes mistaken for a period.
If your due date was set by ultrasound, working back from it gives a notional period date that may not match your real one. That doesn't matter for dating — the due date itself is what counts.
If you're not pregnant and simply don't remember
There is no way to recover the date without some anchor, so don't guess. Look for clues: phone notes, messages, when you bought pads or tampons. At an appointment, "I don't remember, roughly that week" is a perfectly fine answer.
From your next period, start noting the first day. The Ponyatno app has a My cycle section that keeps the dates, works out your cycle length and tells you when to expect the next one.
This calculator is a reference tool, not a diagnosis. Laboratory ranges differ, and the result depends on your health, medicines and other tests. Discuss it with your doctor.
FAQ
How can I find my last period date if I don't remember it?
If you're pregnant and have had a scan, wind the scan's gestational age back from the scan date — that's the most reliable route. With a due date, subtract 280 days. Otherwise use the conception date or the day your period was due, with a wider margin.
What happens if I can't give my LMP at the first appointment?
Nothing bad. Your pregnancy will be dated by ultrasound, which in the first trimester is more accurate than memory. Give a rough timeframe if you have one.
Why does ultrasound accuracy change with the week?
Early on, embryos grow at almost the same rate, so size points precisely to age. Later, individual differences kick in, so by the late second and third trimester the margin reaches ±14–21 days.
Can I work out the day of conception from this?
Roughly: in a 28-day cycle conception is usually around day 14, in other cycles about 14 days before the next period. Nobody knows the exact day, which is why pregnancy is dated from the period.
My scan dates don't match my own calculation. Which is right?
Usually the period-based date is kept if the difference is small and replaced by the scan date if it exceeds the scan's margin of error. Your doctor or midwife makes that call.
Sources
Formulas and cut-offs follow clinical guidelines — see sources below.
- ACOG Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150–e154.