How to Calculate Weeks Pregnant

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Pregnancy Calculator: Weeks Pregnant

Calculate your current gestational age based on your Last Menstrual Period (LMP).

Your Estimated Gestational Age:

— weeks and — days

(Based on a standard 280-day gestation period)

Understanding How Pregnancy Weeks Are Calculated

Pregnancy dating is a crucial part of prenatal care, helping healthcare providers estimate your baby's development, schedule important tests, and determine your estimated due date (EDD). The most common method used by doctors and midwives is the Naegele's rule, which calculates gestational age based on the first day of your last menstrual period (LMP).

This method assumes a standard pregnancy length of 40 weeks (280 days) from the first day of your LMP, which is considered "full term." While conception typically occurs about two weeks after your LMP, the 40-week count starts from the beginning of your period.

The calculation is straightforward:

  • Duration of Pregnancy: A full-term pregnancy is considered 40 weeks and 0 days (or 280 days) from the first day of your LMP.
  • Current Date vs. LMP: The difference in days between the current date and the first day of your LMP is calculated.
  • Weeks and Days: This total number of days is then divided by 7 to determine the number of full weeks. The remainder represents the additional days.

For example, if your LMP was on January 1st, 2024, and today's date is January 22nd, 2024:

  • The difference is 21 days.
  • 21 days / 7 days/week = 3 weeks and 0 days. So, you would be 3 weeks pregnant.

Our calculator uses this standard method to provide an estimate of your current gestational age.

Why is This Calculation Important?

Knowing your estimated gestational age is vital for several reasons:

  • Monitoring Fetal Development: Gestational age helps track your baby's growth and development milestones against what's expected at each stage.
  • Prenatal Appointments: It guides the timing of your doctor's visits and recommended screenings (like ultrasounds, blood tests, and genetic screenings).
  • Estimating Due Date: While not always exact, the 40-week mark from your LMP provides a general timeframe for your baby's arrival.
  • Preparation: It allows you to prepare for labor and delivery.
Important Note: This calculator provides an estimate based on the standard LMP method. If you have irregular cycles, ovulate later than usual, or are unsure of your LMP date, your actual gestational age might differ. Ultrasound measurements, especially in the first trimester, are often considered more accurate for dating a pregnancy. Always consult with your healthcare provider for the most precise information regarding your pregnancy.
function calculatePregnancyWeeks() { var lmpInput = document.getElementById("lastMenstrualPeriod").value; var resultWeeksElement = document.getElementById("pregnancyWeeks"); var resultDaysElement = document.getElementById("pregnancyDays"); if (!lmpInput) { resultWeeksElement.textContent = "–"; resultDaysElement.textContent = "–"; return; } var lmpDate = new Date(lmpInput); var today = new Date(); // Ensure today's date is after LMP for a valid calculation if (today < lmpDate) { resultWeeksElement.textContent = "N/A"; resultDaysElement.textContent = "N/A"; alert("Today's date cannot be before your Last Menstrual Period date."); return; } // Calculate the difference in milliseconds var timeDifference = today.getTime() – lmpDate.getTime(); // Convert milliseconds to days var daysDifference = Math.floor(timeDifference / (1000 * 60 * 60 * 24)); // Calculate weeks and remaining days var weeks = Math.floor(daysDifference / 7); var days = daysDifference % 7; // Display the result resultWeeksElement.textContent = weeks; resultDaysElement.textContent = days; } // Optional: Automatically calculate when the page loads if a date is pre-filled or on date change // document.getElementById("lastMenstrualPeriod").addEventListener("change", calculatePregnancyWeeks); // You might want to call calculatePregnancyWeeks() here if you want it to run on page load with a default date, but for this use case, a button click is more appropriate.

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