Pregnancy Calculator
Calculate your due date, current gestational age, trimester, and key pregnancy milestones from your last menstrual period (LMP).
| Week | Date | Milestone | Status |
|---|---|---|---|
| Enter your LMP above to see milestones | |||
Calculate your estimated due date, current gestational age, trimester, and upcoming pregnancy milestones from the first day of your last menstrual period (LMP).
How Pregnancy Calculator Works
The calculator applies Naegele's rule: due date = LMP + 280 days (40 weeks), which assumes a standard 28-day cycle with ovulation around day 14. If you enter a different average cycle length, the due date shifts by the same number of days your cycle differs from 28 — a 32-day cycle pushes the estimate 4 days later, for example.
Gestational age — the "weeks pregnant" figure doctors use — is counted from the first day of your LMP, not from the actual date of conception, which is why it runs about two weeks ahead of true fetal age. Conception itself is estimated at LMP + 14 days, adjusted the same way for cycle length.
The milestone table maps out ten key points in pregnancy — from the earliest possible positive test around week 4 to the estimated due date at week 40 — calculated as calendar dates from your specific LMP so you can see exactly when each stage should occur for your pregnancy.
See It In Action
Who Uses Pregnancy Calculator and Why
- Estimating a due date and current week of pregnancy shortly after finding out you're pregnant.
- Checking upcoming milestone dates (like the anatomy scan) to plan appointments and prepare in advance.
- Getting a starting gestational age figure to bring to a first prenatal appointment.
Mistakes to Avoid
- Assuming the calculated due date is a fixed deadline — only about 5% of babies are born on their exact calculated due date, so treat it as an estimate within a normal 38-to-42-week window.
- Entering an LMP date from memory when cycles are irregular, without adjusting the cycle length field — this can shift the estimate by several days or more from what an ultrasound would show.
- Confusing gestational age (counted from LMP) with fetal age (counted from actual conception, about two weeks later) when comparing notes with someone else's pregnancy.
Tips for Best Results
- If your cycles are irregular or you're unsure of your exact LMP, mention that at your first prenatal visit — an early ultrasound is generally more precise than an LMP-based estimate in that situation.
- Use the milestone table to plan around key appointments (like the week-20 anatomy scan) well in advance rather than tracking them manually.
Fixing Common Problems
My due date from this calculator doesn't match what my doctor told me. — A clinical due date is often confirmed or adjusted by ultrasound measurement, which can differ from a pure LMP calculation by several days, especially with irregular cycles — your provider's ultrasound-based estimate takes precedence.
Terms Explained
Naegele's rule: The standard formula for estimating a due date: LMP plus 280 days, assuming a 28-day cycle with ovulation on day 14.
Gestational age: Pregnancy duration counted from the first day of the last menstrual period, not from actual conception — the convention used throughout prenatal care.