Pregnancy Calculator

How Gestational Age & Due Dates are Calculated

A standard human pregnancy lasts approximately 40 weeks (280 days) from the first day of the last menstrual period (LMP). Medical professionals use this timeline because the exact date of conception is rarely known.

Calculation Methods

• Due Date: If you already know your due date, the calculator reverse-calculates your current gestational age and conception dates accordingly.
• Last Menstrual Period (LMP): Calculated using Naegele's Rule. The formula starts with the first day of your last period, adds 280 days, and adjusts for cycle length: $\text{Due Date} = \text{LMP} + 280\text{ days} + (\text{Cycle Length} - 28)\text{ days}$.
• Ultrasound scan: In early pregnancy, an ultrasound scan measuring the crown-rump length is the most accurate way to date a pregnancy. It overrides dates calculated from periods if they differ by more than 5-7 days.
• Conception Date: If the conception date is known, the due date is calculated by adding 266 days directly (which corresponds to 38 weeks of fetal development).
• IVF Transfer Date: IVF transfers have a precise conception age. For a 3-day embryo, due date is 263 days from transfer. For a 5-day blastocyst, due date is 261 days from transfer.

Trimester Classifications

A pregnancy is divided into three trimesters:

• First Trimester: Weeks 1 through 13.
• Second Trimester: Weeks 14 through 27.
• Third Trimester: Weeks 28 through 40 (and up to delivery).