What does LMP mean?
LMP stands for last menstrual period, and specifically the first day of it – the first day of proper bleeding, not spotting. It is the anchor your doctor uses to calculate gestational age and your estimated due date. When your gynaecologist asks “When was your LMP?”, this is the date they want.
Can’t remember your LMP?
You are far from alone – studies suggest a large share of women are unsure of the exact date. Some ways to pin it down:
- Check a period-tracking app, or your phone’s photo and message history around that time.
- Think about events: a festival, a trip, a family function or payday you remember being on your period for.
- If you remember the week but not the day, enter the middle of that week – the result will be within a few days.
- If you have no idea, don’t worry. A dating ultrasound between 7 and 13 weeks will give an accurate date. Use the ultrasound method once you have the report.
Why cycle length matters
LMP dating assumes ovulation two weeks after your period starts. That is only true for a 28-day cycle. Ovulation happens roughly 14 days before your next period, so in a 35-day cycle you ovulate around day 21 – a week later than the standard assumption. Our LMP calculator shifts your due date by the difference between your cycle and 28 days.
| Cycle length | Likely ovulation day | Due date adjustment |
|---|---|---|
| 24 days | Day 10 | 4 days earlier |
| 28 days | Day 14 | No change |
| 32 days | Day 18 | 4 days later |
| 35 days | Day 21 | 7 days later |
Irregular periods, PCOS and breastfeeding
If your cycles vary by more than a week from month to month, if you have PCOS, if you conceived while breastfeeding, or soon after stopping hormonal contraception, LMP dating can be misleading. In these situations, an early scan is much more reliable than any calculation – treat the LMP result as a starting point only.
Working backwards: if your doctor gave you a due date, the Due date tab shows the LMP date that matches it (“LMP equivalent”). That is the date to use in other calculators.
Frequently asked questions
Is LMP the first or last day of my period?
LMP always means the first day of your last period – the first day of full flow.
What if I had spotting before my period?
Count from the first day of normal bleeding, not the spotting. Light bleeding around the time of your expected period can also be implantation bleeding, which is why an early scan is useful if you are unsure.
How many weeks pregnant am I from my LMP?
Count the days from your LMP to today and divide by 7. The whole number is your weeks and the remainder is your days. The calculator does this automatically.
Why is my LMP due date different from my ultrasound due date?
LMP dating assumes you ovulated on a set day of your cycle. If you ovulated earlier or later, the baby will measure bigger or smaller than expected on the scan, and your doctor may use the scan date.
Can I use LMP dating after IVF?
IVF pregnancies are dated from the embryo transfer or egg retrieval date, which is more precise. Use our IVF due date calculator.
Medical disclaimer: The results and information on this page are estimates for general education only. They are not a diagnosis and do not replace advice from your gynaecologist, obstetrician or midwife. If you have pain, bleeding, reduced baby movements or feel unwell, contact your doctor or call 108 immediately. Read the full disclaimer.
Sources & further reading
- American College of Obstetricians and Gynecologists (ACOG). Committee Opinion No. 700: Methods for Estimating the Due Date. acog.org
- World Health Organization. WHO recommendations on antenatal care for a positive pregnancy experience (2016). who.int
- NHS (UK). Pregnancy tests; Ovulation; Your pregnancy and baby guide. nhs.uk