Due Date Calculator

1. The due date is calculated based on the first day of your last menstrual period. The formula is: the due date is the month of your last menstrual period plus 9 (or minus 3), plus 7 days. If your last menstrual period was March 20, 2011, your due date is December 27, 2011.A full-term pregnancy lasts approximately 280 days (about 40 weeks), and delivery within two weeks before or after the due date is considered normal. If your menstrual cycle was irregular or longer than average before pregnancy, it is recommended to rely on ultrasound results; however, ultrasound estimates typically have a margin of error of about one week.

2. Methods for calculating the due date:

① Calculated based on the date of the last menstrual period (LMP).

Add 9 or subtract 3 from the month of your last menstrual period to determine the month of the due date; add 7 to the day of your last menstrual period to determine the day of the due date.

Example: If the last menstrual period was June 1, 2022, the month is 6 + 9, and the day is 1 + 7, so the due date is March 8, 2023.

② Calculation based on the date of first fetal movement

If you cannot recall the date of your last menstrual period, you can estimate the due date based on the date you first felt fetal movements. Fetal movements generally begin between 18 and 20 weeks of pregnancy.

The calculation method is: for first-time mothers, add 20 weeks to the date of first fetal movement; for mothers who have given birth before, add 22 weeks to the date of first fetal movement.

③ Calculation based on the basal body temperature chart

Take the last day of the low-temperature phase on your basal body temperature chart as the ovulation date, then count 264–268 days from that date, or add 38 weeks.

④ Calculation based on ultrasound examination

Doctors can estimate the gestational age and calculate the due date by measuring the biparietal diameter (BPD), crown-rump length (CRL), and femur length during an ultrasound examination (a method commonly used by most doctors for diagnosis).

⑤ Calculation based on the onset of morning sickness

Morning sickness typically begins around the end of the 6th week of pregnancy, which is 42 days after the first day of the last menstrual period. Counting 280 days from this date yields the estimated due date.