Enter whichever date you know — the first day of your last period, the date of conception, or an embryo transfer — and get your estimated due date along with how many weeks and days along you are today. Cycles longer or shorter than 28 days are adjusted for, and key dates like the second and third trimester and full term are listed too.
Obstetric pregnancy weeks are counted from the first day of your last period, not from conception. That's why "4 weeks pregnant" means about two weeks after conception — this calculator uses the same convention.
By Naegele's rule: 280 days (40 weeks) from the first day of your last period. Because it assumes ovulation on day 14, a 35-day cycle pushes the due date a week later.
Only about 5% of babies are born exactly on their due date. Most arrive between 37 and 42 weeks, and the due date marks the middle of that window.
An ultrasound dates the pregnancy from the baby's measurements, which is often more accurate than remembering the last period. Go with the date your clinic set.