Understanding the Pregnancy Week Calculator
A pregnancy week calculator tells you how far along you are — your gestational age in weeks and days — and which trimester you are in. Knowing your current week helps you follow your baby’s development, anticipate appointments and screenings, and understand what changes to expect in your body.
How gestational age is counted
Gestational age is measured from the first day of your last menstrual period, which is about two weeks before conception actually occurs. This is the standard medical convention, so when you are told you are “10 weeks pregnant,” the embryo has been developing for roughly 8 weeks. A full-term pregnancy runs about 40 weeks by this count.
Trimesters at a glance
The first trimester (weeks 1–12) is when major organs form; the second (weeks 13–26) is often the most comfortable and brings the first movements; the third (weeks 27–40) is rapid growth and preparation for birth. Tracking your week keeps these milestones in view and helps you know what is typical at each stage.
Tips & things to know
- •Gestational age starts from your last period, not conception.
- •Each trimester brings distinct developmental milestones.
- •An early ultrasound can confirm or adjust your week count.