Gestational Age Calculator
Use our free Gestational age Calculator to get personalized health results. Based on validated medical formulas and clinical guidelines.
Medical disclaimer: This calculator is provided for educational and informational purposes only and does not constitute medical advice, diagnosis, or treatment. Results are general estimates and may not reflect your individual circumstances. Always consult a qualified healthcare professional before making decisions about your health.
Gestational Age Calculator
Calculator
Adjust values & calculateEnter your values below. Every result is computed in your browser — no data is sent to any server.
Formula: GA = (Current Date - LMP) in weeks and days | EDD = LMP + 280 days | Adjusted for cycle length
Worked example — 21 weeks, 6 days — Second trimester
Formula
GA = (Current Date - LMP) in weeks and days | EDD = LMP + 280 days | Adjusted for cycle length
Gestational age is calculated from the first day of the last menstrual period (LMP). Normal pregnancy = 40 weeks (280 days). If cycle length differs from 28 days, adjust by adding (cycle - 28) days to due date.
Worked Examples
Example 1: LMP March 1, 2024
Problem:LMP: March 1, current date: August 1, 28-day cycle
Solution:Days: 153. GA: 21 weeks, 6 days. Due: Dec 6, 2024
Result:21 weeks, 6 days — Second trimester
Example 2: Longer Cycle Adjustment
Problem:LMP: January 10, 2024, current date: April 10, 2024, average cycle length 35 days (7 days longer than the 28-day default).
Solution:Raw days since LMP = 91. Cycle adjustment = 35 - 28 = +7 days, so adjusted days = 91 - 7 = 84 days = 12 weeks even. Due date shifts to LMP + 287 days (280 + 7) instead of the standard 280.
Result:12 weeks, 0 days — First/Second trimester boundary; due date moved 7 days later than a standard 28-day-cycle estimate
Frequently Asked Questions
How is gestational age calculated?
GA is counted from the first day of the last menstrual period (LMP), even though conception typically occurs ~2 weeks later. This is the standard medical convention.
What if my cycle is longer than 28 days?
If your cycle is 35 days, ovulation likely occurred around day 21 instead of day 14. This shifts the due date by +7 days. The calculator adjusts for this automatically.
What are the key milestones in each trimester?
First trimester (weeks 1–12): organ formation, heartbeat detected around week 6. Second trimester (weeks 13–26): rapid fetal growth, movement felt around week 18–20, anatomy scan around week 20. Third trimester (weeks 27–40): lung maturation, weight gain, baby moves into head-down position. Full term is 39–40 weeks.
What does 'viable' mean in pregnancy?
Fetal viability refers to the gestational age at which a baby has a reasonable chance of survival outside the womb with medical support. This threshold is generally considered to be around 24 weeks. Survival rates at 24 weeks are roughly 50–70% with intensive care, and they improve significantly with each additional week of gestation.
Why does my ultrasound due date differ from my LMP-based due date?
First-trimester ultrasound dating (measuring crown-rump length between about 6 and 13 weeks) is considered more accurate than LMP dating alone because it does not depend on remembering the exact first day of your last period or assuming a textbook 28-day cycle with ovulation on day 14. ACOG guidance is that if the ultrasound date and LMP date disagree by more than 5 to 7 days in the first trimester (or by more than 10 days in the early second trimester), the pregnancy is typically re-dated to the ultrasound estimate, since irregular cycles, late ovulation, or an inaccurate recalled LMP are common sources of error.
What is the difference between gestational age and fetal (conceptional) age?
Gestational age counts from the first day of the last menstrual period, roughly two weeks before conception actually occurs, which is why it is sometimes called 'menstrual age.' Fetal or conceptional age counts from the estimated date of conception itself, so it runs about two weeks behind gestational age at any given point. Nearly all clinical practice, due date calculations, and Gestational Age Calculator use gestational age, but the distinction matters when comparing to some embryology references or IVF timelines, which often use conceptional/embryonic age since the fertilization date is known precisely.
What is the difference between early term, full term, late term and post-term?
Rather than treating everything from 37 weeks as one block, obstetric practice divides the end of pregnancy into bands: early term covers 37 weeks 0 days to 38 weeks 6 days, full term 39 weeks 0 days to 40 weeks 6 days, late term the 41st week, and post-term from 42 weeks 0 days onwards. The distinction exists because neonatal outcomes differ measurably between them, which is why elective timing decisions are usually anchored to the full term window and discussed with your obstetric team.
How is gestational age estimated when the last menstrual period is unknown?
Dating then rests entirely on ultrasound biometry, and the earlier the scan the tighter the estimate. Before about 14 weeks the crown-rump length is used, because embryonic growth is very uniform at that stage. Later, a composite of biparietal diameter, head circumference, abdominal circumference and femur length is used instead, but fetuses diverge in size as pregnancy advances, so the margin of error widens from days in the first trimester to a couple of weeks or more near term. Whichever scan comes first generally sets the dating for the rest of the pregnancy.
How is gestational age assigned when the date of conception is known?
When conception timing is known, from a documented ovulation date or an assisted reproduction cycle, gestational age is derived by adding 14 days to the conceptional age, preserving the convention that dating starts two weeks before conception. For pregnancies conceived by assisted reproductive technology, where the retrieval or transfer date is documented, ACOG advises that this cycle-derived date is used for dating and is not revised by later scan measurements; for a conception date inferred from a natural cycle no such rule applies, and ultrasound redating still follows the usual criteria. Gestational Age Calculator works from menstrual dates, so a known conception date is better entered into a conception-based or IVF dating tool.
Why does an accurate gestational age matter for antenatal screening?
Most antenatal tests are only interpretable inside a defined window, because the measurement or analyte they rely on changes week by week. First-trimester combined screening, which pairs nuchal translucency measurement with maternal serum markers, is performed in a narrow band around 11 to 14 weeks, and the detailed anatomy scan sits in the second trimester. Dating that is off by even a week can push a test outside its valid range, so windows should always be confirmed with your provider.
References
Reviewed for accuracy by Rahul Singh, Health & Wellness Specialist · Editorial policy
Related Calculators
🧮Arterial Age Calculator — Vascular Age vs Real Age
Calculate arterial age with inputs, formulas, and instant results.
🧮Fertility by Age Calculator — Chances by Decade
Calculate fertility by age with inputs, formulas, and instant results.
🧮BMI Calculator for Women — Ranges by Age
Calculate bmicalculator for women with inputs, formulas, and instant results.
🧮BMI Calculator for Older Adults (Age 65+)
Calculate geriatric bmicalculator with inputs, formulas, and instant results.
🧮Breast Milk Calculator
Estimate daily breast milk intake for infants by age and weight.
🧮Baby Formula Calculator
Calculate daily formula intake and number of bottles based on baby age and weight.
🧮Baby Growth Percentile Calculator
Plot baby weight, length, and head circumference on WHO growth charts by age.
🧮Teething Timeline Calculator
Estimate when baby teeth will appear based on age and common eruption patterns.