Pregnancy Week Calculator
Free pregnancy week calculator. Find exactly how many weeks and days pregnant you are today, plus your trimester, from your last menstrual period (LMP).
Quick answer
Weeks pregnant = days since LMP ÷ 7. The first trimester ends at 13 weeks; second at 27 weeks; third at 40 weeks. Pregnancy is conventionally counted from LMP, not conception, so 'week 6' is 4 weeks after conception.
Pregnancy Week Calculator
Pregnancy progress
How it works
Pregnancy is dated from the first day of your last menstrual period (LMP), even though conception happens about 2 weeks later. A full term is 40 weeks. Trimesters: first (1-12), second (13-27), third (28-40).
When to use it
Tracking weekly milestones, knowing when prenatal visits are due, planning maternity leave around the due date.
Common mistakes
Confusing "weeks pregnant" with "weeks since conception." Doctors always use the LMP date, so weeks pregnant runs about 2 weeks ahead of weeks since conception. Use the doctor's number when comparing to anything in your medical chart.
How the pregnancy week calculator works
Enter your LMP (last menstrual period). The calculator computes the number of days between LMP and today, divides by 7 for weeks, and outputs weeks + remaining days (e.g., '23 weeks 4 days'). It also identifies your current trimester (1, 2, or 3), days until due date, and key milestone dates. The convention of counting from LMP is universal in obstetrics — it's what providers and apps use.
When to use it
Quick lookup of your current pregnancy progress between provider visits. Tracking when major milestones happen (anatomy scan at ~20 weeks, glucose screen at 24-28 weeks). Confirming pregnancy stage when comparing notes with providers, friends, or pregnancy apps that may differ on conventions. Filling in pregnancy app inputs that ask 'how far along.'
Common mistakes
- Counting from conception. Pregnancy is conventionally counted from LMP, which is about 2 weeks before conception. Saying 'I'm 6 weeks pregnant' (LMP-based) means 4 weeks since conception.
- Using the date of a positive pregnancy test. Tests usually go positive around 4 weeks LMP — the LMP date is what matters for week calculations, not the test date.
- Treating week numbers as absolute. Cycle length variations, late ovulation, and early ultrasound dating can adjust the official 'gestational age.' Trust your provider's dating once a first-trimester ultrasound is done.
Frequently asked questions
How do I know how many weeks pregnant I am?
Count from the first day of your last menstrual period (LMP) — that's day 0. Each 7 days is one week. The calculator above does the math automatically and outputs current weeks, days, and trimester.
Why are weeks counted from LMP and not conception?
Because LMP is a knowable date — most women track their period. Conception happens around day 14 of a typical cycle, but the exact day is rarely known. LMP-based dating is universal in obstetrics, even though it overstates 'true' pregnancy length by 2 weeks.
How do I calculate my trimester?
First trimester: weeks 1-13. Second trimester: weeks 14-27. Third trimester: weeks 28-40 (and beyond — 'full term' is 39-40 weeks, but 37-42 weeks is normal range). The calculator outputs your current trimester automatically.