The gestational count starts at the LMP by universal medical convention, because the exact date of ovulation and fertilization is rarely known precisely. This means that in the first two gestational weeks, pregnancy has not yet occurred biologically — ovulation and fertilization typically happen in weeks 2–3.
Gestational Weeks Calculator
Exact gestational weeks and days from LMP — with current trimester, progress and estimated due date.
How gestational age is counted — and why it starts before conception.
Gestational age is counted from the first day of the last menstrual period, by universal medical convention. This means that in weeks 1 and 2 the person is not yet pregnant — ovulation and fertilization typically occur in week 2 or 3. A fetus at '12 weeks' is 12 gestational weeks old, not 12 weeks from conception.
Enter your LMP and see the exact gestational weeks with context.
- Enter the date of the first day of your last menstrual period (LMP).
- The calculator shows the exact weeks and days of gestation, the current trimester and percentage progress.
- The milestone timeline shows the gestational events closest to the current week.
How gestational weeks are counted in medicine
Gestational counting begins on the first day of the last menstrual period by international medical convention — not at the date of conception. This means that in weeks 1 and 2, pregnancy has not yet occurred biologically.
In clinical practice, gestational age drives all decisions: first-trimester screening, anatomy scan, threshold of fetal viability (week 24) and start of full term (week 37).
Fetal biometry on first-trimester ultrasound is more accurate than LMP-based calculation for pregnancies with irregular cycles or assisted conception.
Why weeks 12, 20, 24 and 37 are key reference points
Week 12 marks the end of the first trimester: miscarriage risk drops significantly and all major organs are already formed.
Week 20 is the pregnancy midpoint and the time of the anatomy scan, which assesses fetal structure, growth and placental position.
Week 24 is the threshold of fetal viability — with intensive neonatal support, babies born from this week onwards have survival chances.
Week 37 marks the start of full term (37–42 weeks), when the baby is prepared to be born with lower risk of prematurity-related complications.
Sources and references for this tool
These references help contextualize formulas, standards, APIs and limitations used on this page. They do not replace professional validation when a result has legal, financial, medical or operational impact.
- ACOG Committee Opinion — Methods for Estimating the Due DateACOG — Official reference for gestational age and EDD estimation.
- Preterm Birth — WHOWorld Health Organization — WHO definitions of prematurity, full term and gestational milestones.
- Gestational Age — StatPearls (NCBI)NCBI / StatPearls — Clinical basis for gestational week counting and medical conventions.
- Fetal Development — American Pregnancy AssociationAmerican Pregnancy Association — Fetal development milestones timeline week by week.