DATING in Pregnancy Scans: How We Determine the Due Date

One of the first questions expectant parents ask after a positive pregnancy test is, “When is the baby due?”

“The process of establishing this expected delivery date is known as Dating“

Dating in pregnancy refers to calculating the baby’s due date.

In simple terms, pregnancy lasts about 40 weeks from the start of the woman’s last menstrual period (LMP).

The expected delivery date (EDD) helps track fetal growth, schedule essential scans, and plan for delivery.

There are two main ways to determine the due date:

1. Using the Last Menstrual Period (LMP)

For women with regular cycles, the due date is calculated by adding 280 days (40 weeks) to the first day of the last menstrual period. This method assumes ovulation and conception occur around day 14 of a 28-day cycle.

2. Ultrasound Dating Scan

Most of the pregnant woman, there is a NO discrepancy in the delivery date that is  seen during the scan.

However, this LMP based delivery date should be confirmed by a early pregnancy scan (before 13 weeks). This scan is referred to as “Dating scan”.

When there is a major discrepancy between LMP based and scan based delivery date; your doctor gives a corrected expected delivery date (CEDD) based on the dating scan.

“In case of discrepancy between LMP and Scan based calculation

Scan based calculation is preferred“

Early pregnancy vs. Dating scan

Any scan done in the early pregnancy (before 13 weeks); which is used to fix the delivery date is termed as “Dating scan”.

Along with establishing the delivery date, early pregnancy scan also looks at

1. Number of fetuses (Single or Multiple pregnancy)
2. Location of pregnancy (inside or outside the Uterus).
3. Checking the fetal heart rate
4. Screening both the ovaries.

Why is Dating Important?

• Helps track fetal growth and detect any delays.
• Determines the timing for important scans like the NT scan (11-14 weeks); anomaly scan (18-22 weeks) and growth scans in the third trimester.
• Aids in deciding the best time for delivery, especially in high-risk pregnancies.

Can the Due Date Change after every scan?

No, it doesn’t. Expected delivery date has to be confirmed by 13 weeks.

Due date should not be changed afterwards. (dating adjustments are not done after 13 weeks, as fetal growth rate varies).

Consult your doctor/fetal medicine consultant regarding any further query.

Dating is the foundation of pregnancy monitoring.

If you’re expecting, make sure to get an early pregnancy scan for the most accurate due date—it’s the first step in tracking your little one’s journey!