Pregnancy Due Date Calculator

How Pregnancy Due Dates Are Calculated

The most common method for calculating a pregnancy due date is Naegele's rule, named after German obstetrician Franz Naegele. This rule adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). The calculation assumes a 28-day menstrual cycle with ovulation occurring on day 14. While this method is used by the vast majority of healthcare providers worldwide, it's important to understand that only about 5% of babies are actually born on their estimated due date. Most births occur within two weeks before or after the calculated date.

Understanding Gestational Age vs Fetal Age

Gestational age counts from the first day of the last menstrual period, even though conception typically doesn't occur until about two weeks later. This means that at "2 weeks pregnant" by gestational age, conception has just occurred. Fetal age (also called embryonic age) counts from actual conception and is therefore about 2 weeks less than gestational age. Medical professionals use gestational age as the standard because the LMP date is usually known with more certainty than the conception date. When your doctor says you are "12 weeks pregnant," they mean 12 weeks gestational age, or about 10 weeks since actual conception.

The Three Trimesters Explained

The first trimester (weeks 1–13) is when major organ systems develop. The embryo grows from a single cell to a recognizable human form about 7.5 cm long. This period carries the highest risk of miscarriage and is when morning sickness is most common. The second trimester (weeks 14–26) is often called the "golden period" because many early symptoms resolve. The baby grows rapidly, movement becomes noticeable, and the anatomy scan at 20 weeks checks for structural abnormalities. The third trimester (weeks 27–40) is characterized by rapid weight gain for the baby, who grows from about 1kg to an average 3.5kg at birth. The lungs mature, and the baby moves into a head-down position in preparation for delivery.

IVF Due Date Calculation

For IVF pregnancies, the due date calculation is more precise because the exact date of embryo transfer is known. For a Day 5 blastocyst transfer, add 261 days (or subtract 19 days from the transfer date to find the equivalent LMP, then add 280 days). For a Day 3 transfer, add 263 days. Some IVF clinics use the egg retrieval date instead, adding 266 days (same as conception date calculation). IVF due dates tend to be more accurate than LMP-based calculations because there's no uncertainty about ovulation timing.

When Ultrasound Changes Your Due Date

First trimester ultrasounds (before 13 weeks) are the most accurate for dating pregnancies, with a margin of error of about ±5 days. If the ultrasound date differs from the LMP date by more than 7 days, most guidelines recommend adjusting the due date to match the ultrasound. Later ultrasounds are less reliable for dating because babies grow at different rates in the second and third trimesters. Your healthcare provider will use clinical judgment to determine the most accurate estimated due date for your pregnancy.