Understanding Gestational Age and Pregnancy Weeks
Pregnancy duration is measured in gestational weeks, counted from the first day of your last menstrual period (LMP), even though conception typically occurs about 14 days later. A full-term pregnancy is 40 weeks (280 days) from LMP. The first trimester spans weeks 1–12, the second trimester weeks 13–26, and the third trimester weeks 27–40. Knowing your gestational age is important for prenatal care scheduling, interpreting ultrasound measurements, and understanding your baby's developmental milestones. This calculator uses the standard obstetric method based on LMP, or allows you to calculate from a known conception date.
Frequently Asked Questions
Why is pregnancy counted from the last period?
Gestational age is counted from the LMP because the exact date of conception is usually unknown, but the LMP is reliably recalled. This means you are technically "pregnant" for 2 weeks before conception actually occurs. Ultrasound measurements in early pregnancy may adjust the due date if they differ from LMP calculations.
How accurate is my due date?
Only about 5% of babies are born on their exact due date. Most arrive within a 2-week window on either side. A due date is an estimate; births from 37–42 weeks are considered normal. Your healthcare provider will monitor progress and advise on induction if pregnancy extends significantly past the due date.
When should I have my first prenatal appointment?
Most guidelines recommend the first prenatal visit between 8–10 weeks. It typically includes a confirmation of pregnancy, blood tests, blood pressure check, urine test, and discussion of prenatal vitamins. If you have risk factors, your provider may want to see you earlier.