Calculate your pregnancy weeks, due date, and trimester
Pregnancy is typically counted from the first day of your last menstrual period (LMP), even though conception usually occurs about 2 weeks later. This means that during "week 1" and "week 2" of pregnancy, you are not actually pregnant yet. A full-term pregnancy lasts approximately 40 weeks (280 days) from the LMP, which is about 38 weeks from actual conception.
Your due date is calculated using Naegele's rule: add 280 days (or 40 weeks) to the first day of your LMP. However, only about 5% of babies arrive exactly on their due date. Most healthy deliveries occur between weeks 37 and 42.
The most dramatic development period. By week 4, the embryo implants in the uterus. By week 6, the heart begins beating. By week 8, all major organs have started forming. By week 12, the fetus is about 2 inches long. Common symptoms: nausea (morning sickness), fatigue, breast tenderness, frequent urination. This trimester carries the highest risk of miscarriage, which decreases significantly after week 12.
Often called the "golden trimester" because morning sickness usually subsides and energy returns. The baby grows rapidly — by week 20, you may feel first movements (quickening). The anatomy scan ultrasound (around week 20) can reveal the baby's sex. The baby develops fingerprints, can hear sounds, and begins sleeping and waking in cycles. Common symptoms: growing belly, back pain, round ligament pain, skin changes.
The home stretch. The baby gains most of its weight during this period. By week 28, the eyes open. By week 32, bones are fully formed but still soft. By week 37, the pregnancy is considered "early term." The baby drops lower in the pelvis in preparation for birth. Common symptoms: Braxton Hicks contractions, shortness of breath, swelling, difficulty sleeping, frequent urination.