妊娠週数・出産予定日計算機

Pregnancy calculations help determine important dates and milestones during pregnancy. This calculator provides accurate estimations based on different calculation methods used by healthcare providers worldwide.

Gestational Age

Gestational age is calculated from the first day of the last menstrual period (LMP). This is the standard method used by healthcare providers worldwide because it's more reliable than trying to pinpoint the exact conception date.

Gestational age is typically 2 weeks longer than fetal age because it includes the 2 weeks before conception.

Calculation Methods

This calculator supports three different methods for calculating pregnancy dates:

• Last Menstrual Period (LMP): Most common method, adds 280 days (40 weeks) to the first day of your last period
• Expected Delivery Date (EDD): Works backward from a known expected delivery date to calculate other important dates
• Conception Date: Adds 266 days (38 weeks) to the estimated conception date

Pregnancy Trimesters

Pregnancy is divided into three trimesters, each with distinct characteristics:

• First Trimester: Weeks 1-12 - Major organ development, morning sickness common
• Second Trimester: Weeks 13-26 - Often called the 'golden period', energy returns
• Third Trimester: Weeks 27-40 - Rapid growth, preparation for birth

Expected Delivery Date (EDD) Accuracy

The Expected Delivery Date (EDD) is an estimate based on a 40-week pregnancy from the last menstrual period. However, only about 5% of babies are born on their exact expected delivery date.

Full-term pregnancy is considered anywhere from 37 to 42 weeks. Most babies are born within 2 weeks of their expected delivery date.

Factors Affecting Accuracy

Several factors can affect the accuracy of pregnancy calculations:

• Menstrual cycle length variations can affect ovulation timing
• Ovulation doesn't always occur exactly 14 days after the last period
• Each pregnancy is unique and may vary from standard calculations

Important Limitations

This calculator provides estimates and should not replace professional medical care:

• Always consult with your healthcare provider for accurate pregnancy dating
• Women with irregular cycles may need ultrasound dating for accuracy
• Pregnancy complications can affect timing and should be monitored by professionals