Two lines on a test, and the first question is always the same: when is the baby due? Enter the first day of your last period and get the estimated due date plus your current week of pregnancy.
How the estimate works
Standard Naegele's rule: last period + 280 days = due date, which is 40 weeks. Gestational age counts from the last period, about two weeks before actual conception — so "4 weeks pregnant" means conception happened roughly two weeks ago. If you know the conception date (common with IVF transfers), that date + 266 days is more precise. Babies are full term between 37 and 42 weeks; only about 5% arrive exactly on the due date.
The three trimesters
First trimester (weeks 1–12): organs form — prenatal vitamins, avoid alcohol and unprescribed medications, schedule the first appointment. Second (13–27): the stable stretch, with the anatomy scan around weeks 18–22. Third (28–40): daily fetal movement counts from week 28, hospital bag, and a birth plan conversation with your provider.