Gestational Age Calculator

Gestational age is the standard way of measuring pregnancy duration, counted from the first day of the last menstrual period (LMP), not from conception. This convention means gestational age at conception is approximately 2 weeks (since ovulation typically occurs ~14 days after LMP).

Gestational age from LMP:

Gestational Age (weeks) = (Today’s Date − LMP Date) ÷ 7

Due date (Naegele’s Rule):

Due Date = LMP + 280 days (40 weeks)

Or equivalently:
Due Date = LMP + 9 months + 7 days

Gestational age from ultrasound (crown-rump length in first trimester):

Gestational Age (days) = 8.052 × √CRL (mm) + 23.73 (Robinson and Fleming, 1975, valid up to a CRL of 84 mm, about 14 weeks)

A 45 mm embryo works out at 8.052 × 6.708 + 23.73 = 77.7 days, which is 11 weeks and 1 day. The quick rule of thumb, CRL in mm plus 42, only holds before about 9 weeks.

Where:

• LMP: first day of last menstrual period (the standard reference point)
• 280 days: average human gestation (40 weeks from LMP)
• CRL: crown-rump length measured by ultrasound in millimeters
• Trimester breakdown: first trimester = weeks 1–13; second = weeks 14–27; third = weeks 28 to birth

Key gestational milestones:

• Week 4: Positive pregnancy test
• Week 6–8: Heartbeat detectable by ultrasound
• Week 10: Cell-free DNA screening can start
• Week 11 to 13+6: First trimester screening (nuchal translucency)
• Week 20: Anatomy ultrasound scan
• Week 24: Viability threshold (can survive outside womb with intensive care)
• Week 37: Early term begins
• Week 39: Full term begins, running to 40 weeks 6 days
• Week 40: Estimated due date
• Week 41: Late term, and week 42 onward is post-term

Until 2013 everything from 37 to 42 weeks was simply “term”. ACOG (the American College of Obstetricians and Gynecologists) split it because babies born at 37 and 38 weeks have more breathing and feeding problems than babies born at 39 or 40. So week 37 is no longer called full term, here or on the due date calculator.

Worked example:

LMP was February 10, 2026. Current date: April 8, 2026.

• Days elapsed: Feb 10 → Apr 8 = 18 days (Feb) + 31 days (Mar) + 8 days (Apr) = 57 days
• Gestational age: 57 ÷ 7 = 8 weeks and 1 day
• Due date: February 10 + 280 days = November 17, 2026

A first-trimester scan dates a pregnancy more accurately than a period date, but a small gap between the two is expected. ACOG re-dates the pregnancy to the scan when they differ by more than 5 days before 9 weeks, or by more than 7 days from 9 weeks to 13 weeks 6 days.