Pregnancy due date (EDD) explained: LMP, conception date, and cycle length
This calculator estimates your due date (EDD) using standard pregnancy dating methods. Use it for planning and understanding your weekly timeline—then confirm medical decisions with your clinician.
Pregnancy weeks calculator (from last period / LMP)
Many people search for a pregnancy weeks calculator to answer: “How many weeks pregnant am I from my last period?” Clinically, gestational age is usually counted from the first day of your LMP.
Rule of thumb: weeks pregnant ≈ (today − LMP) ÷ 7.
Example: LMP on Dec 1, 2025 → on Feb 21, 2026 you’re about 11 weeks + 5 days pregnant (gestational age).
Two common methods: LMP vs conception date
| Method | When it’s used | How it works (simplified) |
|---|---|---|
| LMP (Last Menstrual Period) | Most common clinical method | Due date ≈ LMP + 280 days (40 weeks), adjusted for cycle length |
| Conception date | If you know ovulation/IVF timing | Due date ≈ conception + 266 days (38 weeks) |
Why the calculator may say “4 weeks” when you just missed a period
Gestational age is counted from LMP, not conception. In a typical 28‑day cycle, ovulation happens around day 14—so the embryo is often ~2 weeks “younger” than gestational age.
If you’re tracking fertility, the ovulation calculator can help estimate fertile windows.
Trimester timeline (quick table)
Definitions can vary slightly by country/clinic. Follow your prenatal care team’s guidance.
| Trimester | Weeks (common definition) | What’s changing |
|---|---|---|
| First | Weeks 1–12 | Early development, dating often refined by ultrasound |
| Second | Weeks 13–27 | Growth and anatomy milestones |
| Third | Weeks 28–40 | Rapid growth, preparation for birth |
Interesting due date facts (quick wins)
- Only a small percentage of births happen exactly on the due date.
- Cycle length matters because ovulation timing shifts gestational dating.
- Early ultrasound can refine dating when LMP is uncertain or cycles are irregular.