How is the Estimated Due Date Calculated?
This calculator uses a modified version of Naegele's rule, the standard formula used by obstetricians and gynecologists worldwide to estimate the expected date of delivery (EDD).
Naegele's rule assumes a standard 28-day menstrual cycle with ovulation occurring on day 14. It calculates your due date by adding exactly 280 days (40 weeks) to the first day of your Last Menstrual Period (LMP). Because cycle lengths can vary significantly from woman to woman, our calculator allows you to adjust your average cycle length, which modifies the ovulation and due dates accordingly.
Understanding the Trimesters
A typical pregnancy is roughly 40 weeks long and is divided into three distinct phases, known as trimesters. Each trimester brings its own set of fetal developments and physical changes for the mother.
The First Trimester (Weeks 1 to 12)
Although you may not even know you are pregnant for the first few weeks (since pregnancy technically dates back to your last period), the first trimester is a period of rapid development. Crucial organ systems—including the brain, spinal cord, and heart—begin to form. Many women experience early pregnancy symptoms such as morning sickness, fatigue, and breast tenderness during this time.
The Second Trimester (Weeks 13 to 27)
Often referred to as the "honeymoon phase" of pregnancy, many of the early uncomfortable symptoms fade away during the second trimester. Your energy levels may return, and your "baby bump" will likely begin to show. Sometime between weeks 16 and 20, you may feel the baby's first movements (quickening).
The Third Trimester (Week 28 to Delivery)
The final stretch of pregnancy is focused on the baby's rapid weight gain and the maturation of the lungs. As the baby grows and takes up more space, you may experience physical discomforts such as backaches, shortness of breath, and frequent urination. You will be seeing your healthcare provider frequently as you prepare for labor and delivery.
Is the Due Date Accurate?
It is important to remember that an Estimated Due Date (EDD) is exactly that—an estimate. In reality, only about 4% to 5% of babies are actually born on their exact due date. A pregnancy is generally considered "full-term" if the baby is delivered anywhere between 37 weeks and 42 weeks.