# Gestational Age Calculator

> Gestational age counts from the first day of your last menstrual period. Enter LMP for weeks and days pregnant.

**Last updated:** September 2026
**URL:** https://bodyhealthcalculator.com/gestational-age-calculator
**Category:** Pregnancy & Reproductive Health

## How it works

Gestational age = days since LMP. Full term = 37 to 42 weeks.

## Gestational age formulas

From LMP, gestational age in days = assessment date - LMP date. From EDD, gestational age in days = 280 - days remaining until EDD. Convert total days by taking the whole-number result after division by seven as weeks and the remainder as days.

An EDD is 40 weeks 0 days, or day 280, by convention. If the assessment date is 50 days before the EDD, gestational age is 230 days. That converts to 32 weeks 6 days because 32 times 7 is 224, with 6 days remaining.

Dates after the EDD continue forward. Seven days after the EDD is 41 weeks 0 days, not 40 weeks 7 days.

Dates before the LMP produce no clinically meaningful gestational age and usually indicate an input error.

*Equivalent points on the obstetric calendar*

| Total gestational days | Weeks plus days | Calendar meaning |
| --- | --- | --- |
| 0 | 0 weeks 0 days | First day of LMP |
| 70 | 10 weeks 0 days | First trimester |
| 98 | 14 weeks 0 days | Start of second trimester |
| 196 | 28 weeks 0 days | Start of third trimester |
| 259 | 37 weeks 0 days | Start of early term |
| 280 | 40 weeks 0 days | Estimated due date |

Sources: [1] [3]

## Gestational age versus fetal age

Obstetric gestational age starts at LMP, about two weeks before ovulation in a typical 28-day cycle. Conceptual or fetal age starts near fertilization. A pregnancy labeled 8 weeks gestational age therefore has an embryo that has usually developed for about 6 weeks.

The two-week difference is a convention, not a correction that should be subtracted from every pregnancy. Late ovulation can make the gap larger, and early ovulation can make it smaller.

IVF charts still use gestational age by adding two weeks to embryo age so all prenatal care follows one scale. Medical records, ultrasound reports, screening windows, and delivery guidance use gestational age unless they explicitly say post-fertilization age. Keep the units and reference point when comparing dates from different sources.

Sources: [1] [2] [3]

## Ultrasound dating and redating

ACOG considers crown-rump length through 13 weeks 6 days the most accurate routine dating method, with accuracy around plus or minus 5 to 7 days. Composite fetal measurements carry wider error later: about 7 to 10 days through 21 weeks 6 days, 10 to 14 days through 27 weeks 6 days, and 21 to 30 days in the third trimester.

Clinical redating uses thresholds rather than replacing LMP whenever numbers differ. More than 5 days before 9 weeks or more than 7 days from 9 weeks through 13 weeks 6 days supports a change. Later cutoffs widen to more than 10, 14, or 21 days by gestational period.

A reliable early EDD generally stays fixed. Redating a small third-trimester fetus can conceal growth restriction, while redating a large fetus can conceal excessive growth.

Later scans assess size and interval growth against the established age.

*Ultrasound timing, expected accuracy, and ACOG redating threshold*

| Gestational age | Expected dating accuracy | Difference from LMP supporting redating |
| --- | --- | --- |
| Through 8 weeks 6 days | About 5 to 7 days | More than 5 days |
| 9 weeks to 13 weeks 6 days | About 5 to 7 days | More than 7 days |
| 14 weeks to 15 weeks 6 days | About 7 to 10 days | More than 7 days |
| 16 weeks to 21 weeks 6 days | About 7 to 10 days | More than 10 days |
| 22 weeks to 27 weeks 6 days | About 10 to 14 days | More than 14 days |
| 28 weeks or later | About 21 to 30 days | More than 21 days, with caution |

Sources: [1] [2] [5]

## Trimester and term categories

The first trimester ends at 13 weeks 6 days. The second runs from 14 weeks 0 days through 27 weeks 6 days, and the third begins at 28 weeks 0 days. These boundaries describe gestational age, not fetal size or symptom severity.

Birth before 37 weeks 0 days is preterm. NICHD and ACOG divide term pregnancy into early term at 37 weeks 0 days through 38 weeks 6 days, full term at 39 weeks 0 days through 40 weeks 6 days, late term at 41 weeks 0 days through 41 weeks 6 days, and postterm at 42 weeks 0 days or later.

Calling every pregnancy after 37 weeks full term erases clinically meaningful differences in neonatal risk.

A calculator labels the category but cannot determine whether delivery at a given age is indicated.

Sources: [4] [6]

## How clinicians use gestational age

Gestational age controls the timing and interpretation of prenatal screening, anatomy assessment, fetal growth, glucose testing, preterm treatment, and late-pregnancy surveillance. A date error can place a laboratory value against the wrong reference distribution or make appropriate growth appear abnormal.

Specific schedules vary by history and practice. The anatomy scan commonly occurs around 18 to 22 weeks, and many practices screen for gestational diabetes around 24 to 28 weeks. These are care windows, not permission to delay prenatal contact until a milestone.

CDC birth records use the best obstetric estimate because LMP-only calculations can misclassify preterm and postterm births.

Public-health categories and an individual treatment plan should use the same documented clinical date.

Sources: [1] [7] [8]

## Irregular cycles, uncertain LMP, and IVF

LMP age is weak when cycles are irregular, the bleeding date is uncertain, hormonal contraception recently stopped, or early-pregnancy bleeding resembled a period. Cycle-length correction may improve a stable non-28-day cycle, but it cannot solve unpredictable ovulation.

For IVF, use transfer date and embryo age. A day-5 transfer is already 2 weeks 5 days gestational age; a day-3 transfer is 2 weeks 3 days. The EDD is transfer date plus 261 days for day 5 or plus 263 days for day 3.

Use the fertility clinic's EDD in donor-egg, gestational-carrier, fresh-transfer, and frozen-transfer pregnancies.

The carrier's LMP may reflect treatment hormones and should not replace the laboratory timeline.

Sources: [1] [2] [9]

## Multiples and suboptimally dated pregnancy

Twins and higher-order multiples use one gestational age for the pregnancy. Early ultrasound establishes dating and chorionicity. Later differences in fetal size are monitored as growth differences, not assigned as separate ages.

ACOG defines a pregnancy without ultrasound confirmation or revision before 22 weeks 0 days as suboptimally dated. Elective delivery is not recommended in that setting. Clinicians use the best estimate, may repeat ultrasound in 3 to 4 weeks, and may consider surveillance at 39 to 40 weeks because the true age could be greater.

A fundal height or late ultrasound can support assessment but does not recover first-trimester precision.

Fibroids, body habitus, amniotic-fluid volume, fetal position, and multiple gestation can alter size-based estimates.

Sources: [1] [5] [10]

## Interpretation, uncertainty, and safety

The displayed age is exact arithmetic from an estimated anchor.

If the anchor carries a seven-day uncertainty, a precise output such as 18 weeks 2 days does not remove that uncertainty. Keep the source date and its confidence beside the result.

This calculator cannot assess fetal growth, viability, cervical change, labor, or whether a preterm intervention is appropriate. Contact the maternity unit for fluid leakage, vaginal bleeding, regular painful contractions, severe headache, vision changes, chest pain, breathing difficulty, or reduced fetal movement.

Use the official EDD in every app and form after clinicians finalize dating. Ask the care team to reconcile conflicting dates instead of averaging them. Medication, screening, corticosteroid, induction, and delivery decisions require clinical assessment.

Calendar day handling matters near midnight and across time zones. Clinical gestational age advances by one day at a time according to the date used by the care system, not by counting partial 24-hour intervals from a remembered time of bleeding. Record dates in the local calendar used for appointments.

Leap years do not change the formula. Date software should count the actual February 29 when it falls between the anchor and assessment date. Hand calculations that assume every month has 30 days can be wrong, so count calendar days or use the established EDD method.

Neonatal teams may assess maturity after birth, but a newborn examination is less precise than reliable early pregnancy dating. It should not be used casually to rewrite the prenatal record. The best obstetric estimate remains the standard for clinical care and birth reporting.

When communicating an age, include the assessment date. Saying 24 weeks without a date becomes stale the next day and can be misunderstood when records cross practices.

Write 24 weeks 3 days on October 8, based on the official EDD, so another clinician can reproduce the calculation.

*Gestational age categories. ACOG definitions.*

| Gestational age | Category |
| --- | --- |
| Before 37 weeks 0 days | Preterm |
| 37 weeks 0 days to 38 weeks 6 days | Early term |
| 39 weeks 0 days to 40 weeks 6 days | Full term |
| 41 weeks 0 days to 41 weeks 6 days | Late term |
| 42 weeks 0 days and beyond | Postterm |

Sources: [1] [5] [11]

## FAQ

### Gestational age vs fetal age?

Gestational age includes ~2 weeks before conception. Fetal age is ~2 weeks less.

## References

1. ACOG. [Methods for Estimating the Due Date](https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date)
2. NIH/NLM. [Pregnancy Dating](https://www.ncbi.nlm.nih.gov/books/NBK442018/)
3. NIH/NLM. [Estimated Date of Delivery](https://www.ncbi.nlm.nih.gov/books/NBK536986/)
4. NICHD. [Know Your Terms](https://www.nichd.nih.gov/ncmhep/initiatives/know-your-terms/health-care-providers)
5. ACOG. [Management of Suboptimally Dated Pregnancies](https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/03/management-of-suboptimally-dated-pregnancies)
6. ACOG. [Definition of Term Pregnancy](https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2013/11/definition-of-term-pregnancy)
7. NICHD. [Prenatal Care](https://www.nichd.nih.gov/health/topics/pregnancy/conditioninfo/prenatal-care)
8. CDC. [Obstetric Estimate of Gestation at Delivery](https://www.cdc.gov/nchs/nvss/facility-worksheets-guide/30.htm)
9. Tunón et al.. [Gestational Age in Pregnancies Conceived After In Vitro Fertilization](https://pubmed.ncbi.nlm.nih.gov/10776011/)
10. NIH/NLM. [Sonography Assessment of Gestational Age](https://www.ncbi.nlm.nih.gov/books/NBK570610/)
11. CDC. [Urgent Maternal Warning Signs](https://www.cdc.gov/hearher/maternal-warning-signs/index.html)

## Related

- [due date calculator](https://bodyhealthcalculator.com/due-date-calculator.md)
- [pregnancy weight gain calculator](https://bodyhealthcalculator.com/pregnancy-weight-gain-calculator.md)
- [conception calculator](https://bodyhealthcalculator.com/conception-calculator.md)