How is pregnancy due date calculated?

Due date is calculated as 280 days (40 weeks) from the first day of your last menstrual period (LMP). This is called Naegele's rule: add 7 days to LMP, subtract 3 months, add 1 year. Only about 5% of babies are born on their exact due date; most arrive within 2 weeks before or after.

What are the three trimesters of pregnancy?

First trimester: weeks 1-12 (major organ development, highest miscarriage risk). Second trimester: weeks 13-26 (baby grows rapidly, mother typically feels best). Third trimester: weeks 27-40 (baby gains weight, prepares for birth). Each trimester is approximately 13-14 weeks.

How accurate is an ultrasound due date vs LMP?

First-trimester ultrasounds are most accurate, typically within 5-7 days. LMP calculations assume a 28-day cycle with ovulation on day 14, which varies between women. If ultrasound date differs by more than 7 days from LMP date, doctors often use the ultrasound date.

What is gestational age vs fetal age?

Gestational age counts from the first day of the last menstrual period and is used for pregnancy dating (40 weeks total). Fetal age (conceptional age) counts from actual conception, which is about 2 weeks less. When doctors say "10 weeks pregnant," they mean 10 weeks gestational age.

What milestones happen each trimester?

First trimester: heartbeat detectable (6 weeks), all major organs forming. Second trimester: can feel movement (16-25 weeks), gender visible on ultrasound (18-20 weeks). Third trimester: baby viable outside womb (24+ weeks), rapid brain development, baby moves to head-down position.

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Pregnancy Calculator

Pregnancy due date is calculated as 280 days (40 weeks) from the first day of the last menstrual period. Pregnancy is divided into three trimesters: weeks 1-12, 13-26, and 27-40.

Calculate your pregnancy due date based on last menstrual period, conception date, or known due date. Track milestones and trimester progress.

Calculation Method

Last Menstrual Period

First day of your last menstrual period

Important Notes

This calculator provides estimates based on a 40-week pregnancy
Only about 5% of babies are born on their due date
Always consult with your healthcare provider for accurate dating
Ultrasound dating is more accurate than LMP-based calculations

How to Use

Enter your value in the input field
Click the Calculate/Convert button
Copy the result to your clipboard

Frequently Asked Questions

How is pregnancy due date calculated?: Due date is calculated as 280 days (40 weeks) from the first day of your last menstrual period (LMP). This is called Naegele's rule: add 7 days to LMP, subtract 3 months, add 1 year. Only about 5% of babies are born on their exact due date; most arrive within 2 weeks before or after.
What are the three trimesters of pregnancy?: First trimester: weeks 1-12 (major organ development, highest miscarriage risk). Second trimester: weeks 13-26 (baby grows rapidly, mother typically feels best). Third trimester: weeks 27-40 (baby gains weight, prepares for birth). Each trimester is approximately 13-14 weeks.
How accurate is an ultrasound due date vs LMP?: First-trimester ultrasounds are most accurate, typically within 5-7 days. LMP calculations assume a 28-day cycle with ovulation on day 14, which varies between women. If ultrasound date differs by more than 7 days from LMP date, doctors often use the ultrasound date.
What is gestational age vs fetal age?: Gestational age counts from the first day of the last menstrual period and is used for pregnancy dating (40 weeks total). Fetal age (conceptional age) counts from actual conception, which is about 2 weeks less. When doctors say "10 weeks pregnant," they mean 10 weeks gestational age.
What milestones happen each trimester?: First trimester: heartbeat detectable (6 weeks), all major organs forming. Second trimester: can feel movement (16-25 weeks), gender visible on ultrasound (18-20 weeks). Third trimester: baby viable outside womb (24+ weeks), rapid brain development, baby moves to head-down position.

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