Pregnancy Due Date & Gestational Age Calculator
Calculate your estimated due date and current progress based on medical standards.
How is Pregnancy Calculated?
Pregnancy is traditionally calculated using the "Gestational Age" method rather than the date of conception. This is because most women do not know exactly when they ovulated, but they do know the start date of their last menstrual period (LMP). Medical professionals count 40 weeks (280 days) from the first day of your last period to determine your Estimated Due Date (EDD).
Understanding Naegele's Rule
Most clinicians use Naegele's Rule to estimate your due date. The formula is simple: add seven days to the first day of your LMP, subtract three months, and add one year. For example, if your LMP was May 1, 2023, you add seven days (May 8), subtract three months (February 8), and add a year, resulting in a due date of February 8, 2024.
This calculator improves upon the standard Naegele's Rule by adjusting for your specific menstrual cycle length. Since the standard rule assumes a 28-day cycle, women with longer or shorter cycles require an adjustment to ensure accuracy.
The Three Trimesters
Pregnancy is divided into three distinct phases:
- First Trimester (Weeks 1-13): The crucial period of organogenesis where the baby's fundamental structures form.
- Second Trimester (Weeks 14-26): Often called the "golden period," where many symptoms subside and the baby begins to move.
- Third Trimester (Week 27-Birth): A period of rapid growth as the baby prepares for life outside the womb.
Gestational Age vs. Fetal Age
It is important to note the difference between gestational age and fetal age. Gestational age (what doctors use) begins on the first day of your LMP, meaning for the first two weeks of "pregnancy," you aren't actually pregnant yet. Fetal age is the actual age of the developing baby, which is typically two weeks shorter than the gestational age.