Three ways to calculate — which one to use?
- Last Menstrual Period (LMP) — The most common method. Pregnancy is officially measured from day 1 of your last period, even though conception happened ~2 weeks later. Use this if you know your last period date.
- Conception date — If you tracked ovulation or had IVF, you may know the exact conception date. Most accurate when you do.
- Ultrasound — If you've already had a first-trimester ultrasound, use the gestational age your doctor gave you (e.g., "8 weeks 3 days"). This is the most precise method, especially before 12 weeks.
Pregnancy = 40 weeks from LMP, not from conception
This trips up almost everyone. The "40 weeks" includes ~2 weeks before you actually got pregnant. So when you're "6 weeks pregnant," conception was about 4 weeks ago.
What the trimester milestones tell you
- End of 1st trimester (week 13) — Morning sickness usually fades. Miscarriage risk drops sharply.
- End of 2nd trimester (week 27) — Baby's organs are largely formed; outside-the-womb survival becomes plausible (with intensive care) from here on.
- Week 37 — Officially "full term." Most babies born from here are healthy.
- Week 40 — Estimated due date. Only ~5% of babies are actually born on this exact day.
The due date is an estimate, not a deadline
Healthy full-term babies arrive anywhere between weeks 37 and 42. Doctors typically don't induce until at least 41 weeks unless there's a medical reason.
FAQ
Does cycle length affect the due date?
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Yes — the standard formula assumes a 28-day cycle. If yours is consistently longer or shorter, ovulation happens later or earlier, which shifts the conception date. This calculator adjusts for it automatically when you set your cycle length.
How accurate is the date from LMP?
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About ±5 days for someone with a regular 28-day cycle. Less accurate if cycles are irregular. Ultrasound dating before 12 weeks is more accurate than LMP for most people.
Is this a substitute for prenatal care?
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No. This is for informational purposes only. Always confirm dates and milestones with your OB-GYN or midwife. They have access to your full clinical picture.