How are pregnancy weeks counted?
Pregnancy weeks are counted from the first day of your last menstrual period (LMP), not from conception. This means the first 2 weeks are technically before fertilization occurs. This calculator estimates your current week, trimester, and estimated due date.
If your LMP was 20 weeks ago, you are in week 21 (second trimester), halfway through the pregnancy. Your estimated due date would be 20 weeks ahead (40 total weeks from the LMP).
Formula
EDD = LMP + 280 days (40 weeks)
Step-by-step example
- First day of last menstrual period (LMP) = January 1
- Current week: weeks elapsed since the LMP
- Estimated due date = January 1 + 280 days = October 8
- Current trimester based on the week
The three trimesters
First trimester (weeks 1-12)
- Formation of major organs
- Heartbeat detectable from week 6
- Highest risk of complications
Second trimester (weeks 13-26)
- First fetal movements (weeks 16-20)
- Baby's sex can be determined
- Most comfortable period for the mother
Third trimester (weeks 27-40)
- Complete lung development
- Rapid baby weight gain
- Preparation for birth
Only 5% of babies are born on their exact due date. Full term is considered between weeks 37 and 42.
Important pregnancy milestones
| Week | Milestone | |---|---| | 4 | Embryo implantation | | 8 | Fingers form | | 12 | End of first trimester | | 16 | First movements | | 20 | Halfway through pregnancy | | 24 | Fetal viability | | 28 | Third trimester | | 37 | Full term | | 40 | Estimated due date |
How to read the result
If your cycle is regular, the LMP calculation is reliable; if irregular or the date is unknown, the doctor will adjust with the first ultrasound. Weeks are counted as completed: "20 weeks pregnant" means you are in week 21. The normal birth window is wide (37-42 weeks), so the EDD is an estimate, not a promise.
This calculator is for reference and assumes regular 28-day cycles. The real due date is confirmed by ultrasound and medical evaluation. For any doubt or symptom, consult your doctor.
Frequently asked questions
Because the LMP date is objective and most people remember it, while the exact conception date is hard to determine. Thus, weeks 1-2 exist before fertilization.
40 weeks from the LMP (280 days). Full term is between weeks 37 and 42; only 5% are born on the exact estimated date.
The LMP calculation is less accurate. Your doctor will adjust gestational age with the first-trimester ultrasound, the most reliable method.
Usually between weeks 16 and 20 in a first pregnancy, and slightly earlier (14-16) in later pregnancies.
Related calculators
- Due date: calculate your estimated due date
- Ovulation calculator: estimate conception
- Date calculator: count days and weeks between dates
- Pregnancy weight gain: recommended range by trimester