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Calculation Machine

Gestational Age Calculator

Weeks pregnant from LMP.

Reviewed by the Calculation Machine team · How we check our maths

Your values

ⓘ Live results

Results
Gestational age
18 weeks 0 days
Trimester
Second
Days remaining to 40 weeks
154
Percent of pregnancy complete
45%
Estimated fetal age
16 weeks

Quick answer

Gestational age counts from the first day of your last period, so it runs about two weeks ahead of conception.

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Days since last period126

Result

  • Gestational age18 weeks 0 days
  • TrimesterSecond
  • Days remaining to 40 weeks154
  • Percent of pregnancy complete45%
  • Estimated fetal age16 weeks

How to use the Gestational Age Calculator

  1. 1Count days since the first day of your last period.
  2. 2Enter that number.
  3. 3Read your gestational age and trimester.

Frequently asked questions

Why is fetal age two weeks less?

Ovulation usually happens about 14 days after your period starts.

Is an ultrasound more accurate?

Yes, especially a first trimester dating scan.

About this calculator

How gestational age is counted

Gestational age is counted from the first day of the last menstrual period, not from conception, so at the moment of conception a pregnancy is already about two weeks along. That convention is why a full-term pregnancy is described as 40 weeks.

The result is shown in weeks and days (for example, 12w 3d) because many milestones — scans, screening tests, viability — are tied to specific weeks.

Reading the result

  • If you have had a dating scan, use the scan date rather than period dates.
  • The first trimester ends at 13w 6d; the second runs to 27w 6d.
  • Full term is 39w 0d to 40w 6d; 37–38 weeks is early term.

Sources

More questions

Is gestational age the same as fetal age?

No. Fetal (conception) age is about two weeks less than gestational age.

Why does my scan give a different age?

Early scans measure the baby directly and correct for irregular cycles or uncertain dates.