Pregnancy Calculator

Pregnancy due date calculators are typically accurate within a week or two. Most healthcare providers consider a full-term pregnancy to be between 39 weeks and 40 weeks and 6 days. Only about 5% of babies are born on their exact due date. The calculator uses the standard 280-day pregnancy (40 weeks) from the first day of your last menstrual period, but this can vary based on individual factors.

Gestational age is the age of the pregnancy from the first day of your last menstrual period (LMP), while fetal age is the actual age of the fetus from conception. Gestational age is typically 2 weeks longer than fetal age because it includes the two weeks before conception when you weren’t actually pregnant yet. Healthcare providers use gestational age because it’s easier to determine from the LMP date.

If you have irregular periods, calculating your due date based on your last menstrual period may be less accurate. In this case, an early ultrasound is the most reliable method for determining your due date. Ultrasounds in the first trimester (before 13 weeks) are the most accurate for dating a pregnancy, with a margin of error of about 5-7 days.

The three trimesters of pregnancy are divided as follows: First trimester: Week 1 to the end of week 12; Second trimester: Week 13 to the end of week 26; Third trimester: Week 27 to the end of the pregnancy. Each trimester is marked by different developmental milestones for the baby and different changes in the mother’s body.

For IVF pregnancies, the due date calculation is more precise. If a 3-day embryo transfer was performed, the due date is calculated as 263 days from the transfer date. If a 5-day blastocyst transfer was performed, the due date is calculated as 261 days from the transfer date. This method is more accurate than LMP-based calculations because the exact date of conception is known.