How Are Pregnancy Weeks Calculated

Calculate your estimated pregnancy week and due date based on your Last Menstrual Period (LMP) or conception date.

Understanding How Pregnancy Weeks Are Calculated

Calculating pregnancy weeks, also known as gestational age, is a crucial aspect of prenatal care. It helps healthcare providers monitor fetal development, schedule important tests, and estimate your due date. The most common method for determining gestational age is based on your Last Menstrual Period (LMP).

Method 1: Using the Last Menstrual Period (LMP)

This is the standard method used by most healthcare providers. It assumes that conception occurred approximately 2 weeks after the start of your LMP, even though ovulation and conception typically happen later.

The Calculation:

• Pregnancy is considered to start on the first day of your LMP.
• A full-term pregnancy is 40 weeks (280 days) from the first day of your LMP.
• The current gestational age is calculated by counting the number of days from the first day of your LMP to the current date.
• This count is then divided by 7 to get the number of weeks and remaining days. For example, if it's been 10 days since your LMP, you are 1 week and 3 days pregnant.

Formula:
Gestational Age (Weeks) = (Current Date - LMP Start Date) / 7 days

Method 2: Using Estimated Ovulation or Conception Date

If you have a precise ovulation date or know your estimated conception date (e.g., through IVF or careful tracking), this method can provide a more accurate gestational age from that specific point.

The Calculation:

• If using ovulation date: Add 2 weeks to your estimated ovulation date to find your estimated LMP date, then proceed as in Method 1. Or, calculate the days from ovulation to the current date and divide by 7.
• If using conception date: Calculate the number of days from the estimated conception date to the current date and divide by 7. This gives you the "fertilization age" or "conception age."

Formula (using conception date):
Conception Age (Weeks) = (Current Date - Estimated Conception Date) / 7 days

Note: Medical professionals usually report gestational age based on LMP, so a conception age of 2 weeks typically corresponds to a gestational age of 4 weeks.

Estimated Due Date (EDD) Calculation

The estimated due date is typically calculated as 40 weeks (280 days) from the first day of your LMP.

Naegele's Rule: A common shortcut is to:

1. Take the first day of your LMP.
2. Add 7 days.
3. Subtract 3 months.
4. Add 1 year.

(If this results in a date before your LMP, add 12 months instead of subtracting 3.)

Why These Calculations Matter

Accurate dating of pregnancy is vital for:

• Monitoring Fetal Growth: Ensuring the baby is growing at the expected rate.
• Screening Tests: Scheduling important diagnostic tests like the nuchal translucency scan and amniocentesis at the appropriate times.
• Preparing for Birth: Having a reliable estimated due date helps families prepare for the baby's arrival.

This calculator provides an estimation. Always consult with your healthcare provider for the most accurate dating and personalized advice.