Pregnancy due date calculator
Estimate your due date, current week of pregnancy and trimester from the first day of your last period (LMP).
How the due date is calculated
This calculator uses Naegele's rule — the standard method used by the NHS and American College of Obstetricians: 280 days (40 weeks) are added to the first day of your last menstrual period, adjusted for your cycle length if it differs from 28 days. The donut above shows your progress through the standard 40-week pregnancy.
The three trimesters
- First trimester (weeks 1–13) — the embryo's major organs and structures form; morning sickness and fatigue are common.
- Second trimester (weeks 14–27) — often called the "easier" stage; energy tends to return and the bump becomes visible.
- Third trimester (weeks 28–40) — rapid growth, more frequent checkups, and preparation for birth.
Curious exactly how many weeks along you are today? See our how many weeks pregnant am I guide.
Frequently asked questions
How is a due date calculated?
LMP date + 280 days (Naegele's rule), adjusted for cycle length. Only ~5% of babies arrive exactly on the due date.
How many weeks pregnant am I?
Weeks are counted from the first day of your last period — the calculator shows your exact week above.
How accurate is this?
Usually within 1–2 weeks. An early ultrasound gives the most accurate dating — always confirm with your midwife or doctor.
What if my cycle isn't 28 days?
Select your average cycle length above — the calculator adjusts the due date accordingly, since ovulation timing shifts with cycle length.
This tool is for information only and is not medical advice. Always consult your midwife or doctor.