Pregnancy Calculator

Pregnancy Due Date Estimation

Your estimated due date (EDD) is typically calculated from the first day of your last menstrual period (LMP), adding 280 days (40 weeks). CalcSolver's pregnancy calculator provides your due date along with a week-by-week timeline showing which trimester and developmental stage you're in. Understanding how this date is calculated helps you plan prenatal care, schedule important tests, and prepare for your baby's arrival.

Naegele's Rule Explained

The standard method for estimating a due date is Naegele's Rule: add 280 days (or exactly 40 weeks) to the first day of your last menstrual period. In practice, this is often simplified as "subtract 3 months and add 7 days" from the LMP date. For example, if your LMP was January 10, your estimated due date would be October 17. This method assumes a regular 28-day cycle with ovulation on day 14. For longer or shorter cycles, the actual conception date may differ, which is why early ultrasound dating is recommended for confirmation.

Trimester Breakdown

Pregnancy is divided into three trimesters, each lasting approximately 13 weeks. The first trimester (weeks 1–12) is when the embryo develops major organs, the heart begins beating, and the risk of miscarriage is highest. The second trimester (weeks 13–27) is often called the "golden period" — morning sickness subsides, energy returns, and the baby's movements become noticeable. The third trimester (weeks 28–40) involves rapid fetal growth, the baby gaining weight, and preparation for delivery including the baby settling into a head-down position.

Gestational Age vs. Fetal Age

Gestational age and fetal age are two different measurements. Gestational age counts from the first day of your LMP — this is what doctors use and what CalcSolver displays. It is typically 2 weeks longer than fetal age. Fetal age (or embryonic age) counts from the actual date of conception, which is usually about 2 weeks after the LMP. So at 10 weeks gestational age, the baby is approximately 8 weeks old in terms of fetal development. This distinction matters when interpreting ultrasound results and developmental milestones.

Key Milestones by Week

Week 6: The fetal heartbeat can often be detected via ultrasound. Week 8: All major organs begin forming; the embryo is now called a fetus. Week 12: First trimester ends — miscarriage risk drops significantly, and genetic screening tests become available. Week 18–20: The anatomy scan reveals the baby's sex and checks for structural abnormalities. Week 24: The baby reaches viability — survival outside the womb becomes possible with intensive medical support. Week 28: Third trimester begins; the baby's eyes open and brain development accelerates. Week 37: The pregnancy is considered "early term" — delivery is generally safe. Week 40: Full term — your estimated due date arrives.

Real-World Examples

Example 1 — Standard Cycle: Sarah's last period started on March 15. Adding 280 days gives her an estimated due date of December 20. She is currently 20 weeks pregnant, in her second trimester, and her anatomy scan is scheduled for this week.

Example 2 — Irregular Cycle: Maria has a 35-day cycle instead of 28. Her LMP-based due date may be off by about a week. An early ultrasound at 8 weeks confirmed her adjusted due date is one week later than Naegele's rule predicted.

Tips for Expectant Parents

• Track your LMP carefully — the accuracy of your due date depends on knowing this date precisely.
• Schedule your first prenatal visit by week 8 to confirm the pregnancy and get an initial ultrasound.
• Remember that only about 5% of babies arrive on their exact due date — a 2-week window is normal.
• Use CalcSolver's conception calculator to estimate when conception likely occurred.
• Keep a pregnancy journal tracking symptoms, appointments, and milestones week by week.