Your result shows the current gestational age in weeks and days, the estimated due date (EDD), and the current trimester. Gestational age is counted from the first day of the last menstrual period (LMP), which means it includes approximately two weeks before conception actually occurred. A pregnancy at 8 weeks gestational age, for example, represents roughly 6 weeks since fertilization.

The trimester breakdown is clinically important: the first trimester (0-12 weeks) is the period of organogenesis when the fetus is most vulnerable to teratogens; the second trimester (13-26 weeks) is when anatomy scans and screening tests are typically performed; and the third trimester (27-40+ weeks) focuses on fetal growth monitoring and delivery planning. Each trimester has specific milestones, screening tests, and potential complications that clinicians track.

The estimated due date represents 40 weeks (280 days) from the LMP. Only about 5% of babies are born on their actual due date, but approximately 80% are born within two weeks of it. A pregnancy is considered full term between 39 weeks 0 days and 40 weeks 6 days, early term at 37-38 weeks, and post-term beyond 42 weeks.

Use this calculator at any point during pregnancy when you need to determine the current gestational age or expected due date based on the last menstrual period. It is particularly useful at the first prenatal visit to establish dating, which is then confirmed or adjusted by first-trimester ultrasound. Accurate gestational age is essential for scheduling time-sensitive prenatal tests such as the nuchal translucency screen (11-13 weeks), anatomy scan (18-22 weeks), glucose tolerance test (24-28 weeks), and Group B Strep screening (36-37 weeks).

Clinicians also use gestational age to determine fetal viability thresholds, guide decisions about delivery timing in complicated pregnancies, and assess whether fetal growth is appropriate. Patients often find it helpful for tracking pregnancy milestones and understanding which developmental stage their baby has reached.

LMP-based gestational age assumes a regular 28-day menstrual cycle with ovulation on day 14. For women with irregular cycles, cycles longer or shorter than 28 days, or uncertain LMP dates, the calculated gestational age may be inaccurate by one to several weeks. In these cases, first-trimester ultrasound measurement of crown-rump length is more reliable and should be used for dating.

If the ultrasound-based date differs from the LMP-based date by more than 7 days in the first trimester, more than 10 days in the second trimester, or more than 21 days in the third trimester, the ultrasound date is generally adopted as the official estimated due date per ACOG guidelines. This calculator does not incorporate ultrasound data and relies solely on LMP.

The calculator also does not apply to pregnancies conceived through assisted reproductive technology (IVF/ICSI), which have known conception dates and should be dated from the date of embryo transfer. Women who were recently using hormonal contraception may have delayed ovulation after discontinuation, making LMP-based dating unreliable for the first cycle.