Last updated: March 2026
Understanding Your Pregnancy Week
Knowing exactly how far along you are is one of the most common questions during pregnancy. Pregnancy is measured in weeks starting from the first day of your last menstrual period (LMP), which means the clock starts about two weeks before conception actually occurs.
A full-term pregnancy lasts 40 weeks (280 days) from the LMP, or about 38 weeks from conception. This system can be confusing at first — during "week 1" and "week 2," fertilization has not yet happened. But this dating convention is used because the LMP date is usually more reliably known than the exact date of conception.
Each week of pregnancy brings remarkable changes. During the first trimester (weeks 1-12), the embryo develops from a single cell into a recognizable human form with all major organs. By week 12, the fetus is about two inches long and the risk of miscarriage drops significantly.
The second trimester (weeks 13-27) is often considered the most comfortable period. Morning sickness typically subsides, energy returns, and you begin feeling baby's movements — a milestone called quickening that usually happens between weeks 18 and 22. The 20-week anatomy scan provides a detailed look at baby's development.
The third trimester (weeks 28-40) is the home stretch. Baby gains most of their birth weight during these final weeks, and the lungs mature in preparation for breathing air. By week 37, the pregnancy is considered early term, and by week 39, full term.
Keep in mind that due dates are estimates, not deadlines. Only about 5% of babies arrive on their exact due date. Most births occur within a two-week window on either side of the estimated date. Your healthcare provider will monitor your progress and let you know if anything needs attention.
Frequently Asked Questions
How are pregnancy weeks calculated?
Pregnancy weeks are counted from the first day of your last menstrual period (LMP), not from conception. This means during 'week 1' and 'week 2,' you aren't actually pregnant yet — ovulation and fertilization typically happen around week 2-3. A full-term pregnancy is 40 weeks (280 days) from the LMP.
Why does my doctor say a different week than this calculator?
Your doctor may adjust your dates based on an early ultrasound measurement, which can be more accurate than LMP-based dating — especially if your menstrual cycles are irregular. First-trimester ultrasounds are accurate to within about 5-7 days. If your doctor gives you a different due date, use that date in this calculator for the most accurate results.
What are the trimester cutoffs?
The first trimester spans weeks 1-12 (conception through the end of week 12). The second trimester covers weeks 13-27. The third trimester runs from week 28 through delivery, typically around week 40. These divisions roughly correspond to key developmental phases.
How accurate are the baby size comparisons?
The fruit size comparisons are approximate averages used to give parents an intuitive sense of their baby's growth. Actual sizes vary considerably between pregnancies. Your healthcare provider will measure your baby's growth via ultrasound and fundal height to ensure healthy development.
When is a pregnancy considered full term?
A pregnancy is considered early term at 37-38 weeks, full term at 39-40 weeks, late term at 41 weeks, and post-term at 42+ weeks. While babies born at 37 weeks generally do well, research shows that outcomes are best when delivery occurs at 39-40 weeks for uncomplicated pregnancies.