Pregnancy Week Calculator

What is a pregnancy week calculator?

A pregnancy week calculator tells you how far along a pregnancy is right now. Enter a due date and it works out your current week, day within that week, trimester, percentage complete, and how many days remain. It uses the same 280-day model that midwives and obstetricians use worldwide.

How does it work?

Pregnancy is counted as 280 days from the first day of the last menstrual period. Your due date marks day 280. This calculator works backwards from that date to find today's position. Week display runs from 0 to 39 (the number most apps show). Trimesters follow the clinical week: first trimester is weeks 1–13, second is 14–27, third is 28–40.

Why does knowing the week matter?

Every week brings something new - a scan, a milestone, a shift in how your partner feels. Knowing the exact week helps you show up at the right time: planning around appointments, understanding what's normal, and staying connected through all 40 weeks instead of guessing or having to ask.

Frequently asked questions

Everything you need to know about calculating pregnancy weeks and what the numbers mean.

How do I calculate my pregnancy week?

Pregnancy weeks are counted from the first day of your last menstrual period (LMP). A full pregnancy is 280 days (40 weeks). If you know your due date, your LMP-equivalent start is your due date minus 280 days. Divide the days elapsed since that date by 7 to get your current week.

What trimester am I in?

The first trimester spans weeks 1–13 of pregnancy. The second trimester spans weeks 14–27. The third trimester spans weeks 28–40. Enter your due date in the calculator above to see your current trimester instantly.

How many weeks are in a full pregnancy?

A full-term pregnancy is 40 weeks (280 days), counted from the first day of the last menstrual period. Babies born at 37 weeks are considered early term, and babies born at 39–40 weeks are considered full term.

What is my baby the size of this week?

Baby size changes every week. At week 8 your baby is the size of a raspberry (1.6 cm), at week 20 it is the size of a banana (25.6 cm), and at week 40 it is the size of a pumpkin (51.2 cm). Enter your due date in the calculator above to see the exact comparison for your current week.

Is this pregnancy week calculator accurate?

This calculator uses the standard 280-day pregnancy model used by midwives and obstetricians worldwide. It calculates from your confirmed due date (such as one confirmed by a dating scan). For any clinical decisions, always consult your healthcare provider.

What is the difference between pregnancy week and medical week?

The week number shown here (e.g. 'Week 13') is the display week - the number most apps and partners use day-to-day. Clinically, your medical week is one higher. Trimester boundaries use the medical week: T1 ends at medical week 13, T2 at medical week 27.