Pregnancy Week Calculator

How Does the Formula Work?

The pregnancy week calculator determines your current gestational age, trimester, estimated due date, and days remaining based on the first day of your last menstrual period (LMP). This uses Naegele's Rule — the standard medical formula used by obstetricians and midwives worldwide since the 19th century. Pregnancy is counted from the LMP rather than conception because the LMP date is known with certainty while the exact conception date is usually uncertain. The calculator shows a visual progress bar tracking your journey through the 40-week pregnancy.

Understanding Pregnancy Weeks

Pregnancy weeks count from the LMP, meaning you are technically considered two weeks pregnant before conception actually occurs (conception typically happens around day 14 of the cycle). This medical convention exists because it provides a consistent starting point. When your doctor says you are 8 weeks pregnant, the embryo is actually about 6 weeks old. This two-week offset is normal and accounted for in all medical guidelines and development milestones. The 40-week pregnancy divides into three trimesters, each with distinct developmental stages and health considerations for both mother and baby.

First Trimester (Weeks 1-13)

The first trimester is the period of most rapid development. By week 5 the heart begins to beat. By week 8 all major organs have started forming. By week 12 the fetus is fully formed in miniature — fingers, toes, and facial features are present. Common symptoms include nausea (morning sickness affects 70-80 percent of pregnancies), fatigue, breast tenderness, and frequent urination. The first prenatal visit typically occurs between weeks 6 and 8, including an ultrasound to confirm the pregnancy and estimate due date. Prenatal vitamins with folic acid (400-800 mcg daily) are essential during this period to prevent neural tube defects. The risk of miscarriage is highest in the first trimester and decreases significantly after week 12. ACOG (American College of Obstetricians and Gynecologists) recommends scheduling the first prenatal appointment as early as possible.

Second Trimester (Weeks 14-26)

Often called the most comfortable trimester, many women experience relief from first-trimester symptoms. The anatomy scan ultrasound at weeks 18-22 checks for structural abnormalities and often reveals the baby's sex. The mother begins to feel fetal movements (quickening) typically between weeks 16 and 22 — first-time mothers may not feel movement until closer to week 22. The baby grows from about 8 centimeters to 35 centimeters during this trimester. Glucose screening for gestational diabetes typically occurs at weeks 24-28. Weight gain becomes more noticeable — the recommended total pregnancy weight gain varies from 11 to 16 kilograms depending on pre-pregnancy BMI. Regular prenatal visits occur monthly during the second trimester.

Third Trimester (Weeks 27-40)

The final stretch focuses on fetal maturation — lungs develop surfactant for breathing, the brain grows rapidly, and the baby gains significant weight. Common maternal symptoms include back pain, shortness of breath, Braxton-Hicks contractions, and difficulty sleeping. Prenatal visits increase to every two weeks from week 28 and weekly from week 36. The baby typically moves into head-down position by weeks 32-36 in preparation for birth. Full term is officially defined as weeks 39-40, though healthy babies can be born from week 37 onward. The Group B Streptococcus screening occurs at weeks 35-37. Hospital bag preparation and birth plan discussions should happen during this trimester.

Accuracy and Ultrasound Dates

The LMP-based calculation assumes a 28-day cycle with ovulation on day 14. If your cycles are irregular or you are unsure of your LMP date, an early ultrasound (weeks 8-12) provides the most accurate dating. The crown-rump length measurement in early pregnancy estimates gestational age within plus or minus 5 days. If the ultrasound date differs from the LMP date by more than 7 days, your provider may adjust the due date. Only about 5 percent of babies arrive on their exact due date — most are born within a two-week window around the estimated date. This calculator provides the standard LMP-based estimate that serves as the starting reference point for your pregnancy journey.

Common Pregnancy Milestones

Throughout pregnancy, certain milestones mark important developments. Week 6 brings the first heartbeat visible on ultrasound. Week 12 marks the end of the highest-risk period for miscarriage. The anatomy scan at weeks 18-22 is often the most anticipated appointment, revealing detailed fetal structure and frequently the baby's sex. Week 24 marks viability — the earliest point at which a premature baby has a reasonable chance of survival with medical intervention. Week 28 begins the third trimester and more frequent monitoring. Week 37 marks early term — the baby is considered mature enough for safe delivery. Week 39-40 is full term — the ideal window for birth. Understanding these milestones helps you anticipate what comes next and prepare both emotionally and practically for each stage of your pregnancy journey. This calculator keeps you oriented within the 40-week timeline, always knowing exactly where you are and what lies ahead.

Every pregnancy is unique, but the 40-week framework provides a shared language between you and your healthcare team. Enter your last period date and instantly know where you stand — your week, your trimester, your due date, and how far along you are in this incredible journey.