How pregnancy weeks are calculated
Gestational age is the elapsed calendar time from LMP, shown as completed weeks plus days.
For an established EDD, the calculator estimates the dating LMP as 280 days before that due date. Trimester boundaries follow the cited ACOG definitions.
Limitations and when the estimate may differ
- This result describes gestational age and does not measure fetal development or health.
- An EDD entered in established-due-date mode is treated as the clinical date you provide; the calculator does not revise it.
- Use your maternity-care professional's established gestational age for care decisions.
Important: This calculator provides general date estimates only. It does not diagnose pregnancy, assess fetal health or viability, or provide individualized medical advice.
Medical references
- American College of Obstetricians and Gynecologists (ACOG) — How Your Fetus Grows During Pregnancy
- American College of Obstetricians and Gynecologists (ACOG) — Methods for Estimating the Due Date
- NHS — Pregnancy due date calculator
- Saudi Ministry of Health — Pregnancy Due Date Calculator
Medical methodology last reviewed:
Frequently Asked Questions
What does 18 weeks + 3 days mean?
It means 18 complete gestational weeks have elapsed and you are three days into the next week.
Are pregnancy weeks counted from conception?
No. Clinical convention usually counts from the first day of the last menstrual period.