Ultrasound Due Date Calculator

Weeks Days

Estimated Due Date

Pregnancy Milestones

  • First Trimester (Weeks 1-12) Baby's major organs and systems develop
  • Second Trimester (Weeks 13-26) Baby starts moving, gender can be determined
  • Third Trimester (Weeks 27-40) Baby gains weight and prepares for birth
  • Viability Milestone (Week 24) Baby has a chance of survival outside the womb
  • Full Term (Weeks 37-42) Baby is considered fully developed

Due Date Calculator by Ultrasound: A Complete Guide

Finding out you're pregnant is an exciting moment, and one of the first questions that comes to mind is: When will the baby arrive? Your baby's due date, also known as the Estimated Due Date (EDD), provides a timeline to prepare for the big day. While many expectant parents calculate their due date using the date of their last menstrual period (LMP), ultrasounds offer a more accurate way to determine how far along you are in your pregnancy and estimate your baby's arrival.

This guide explains how due dates are calculated by ultrasound, why they might differ from LMP-based estimates, and what to expect during your scans.

Key Points About Ultrasound Due Dates

  • Due Date Accuracy: Due dates are estimates, not guarantees. Only 5% of babies are born on their actual due date.
  • Normal Range: Full-term pregnancies typically last between 37 and 41 weeks.
  • Changes in Due Date: Your due date may change if the ultrasound measurements don't align with the date of your last period.
  • Medical Importance: Knowing your due date helps healthcare providers monitor your baby's development and plan care during pregnancy.

How is Your Due Date Calculated?

Using the Date of Your Last Menstrual Period (LMP)

  • Pregnancy is typically counted as 40 weeks (280 days) from the first day of your last period.
  • Assumes a regular 28-day menstrual cycle with ovulation occurring on day 14.

Using an Ultrasound Scan

  • Ultrasound uses the size of the baby to determine gestational age.
  • Particularly useful for those with irregular cycles or uncertain LMP dates.

Differences Between LMP and Ultrasound Calculations

Method How it Works When It's Accurate
LMP-Based Due Date Counts 40 weeks from the start of your period Assumes regular 28-day cycles and accurate memory of LMP.
Ultrasound-Based Due Date Measures baby's size via imaging More accurate in early pregnancy (before 12 weeks).

How Ultrasounds Determine Your Due Date

The Dating Scan (First Trimester)

  • Timing: Usually performed between 8 and 12 weeks of pregnancy.
  • Method: Measures the baby's crown-rump length (distance from the head to the bottom).
  • Accuracy: Most reliable way to estimate your due date. Variations in growth are minimal during this stage.

Later Scans (Second or Third Trimester)

  • Timing: Performed after 12 weeks, typically around 20 weeks (anatomy scan) or later.
  • Method: Measures the baby's head, abdomen, and femur length.
  • Accuracy: Less accurate as babies grow at different rates later in pregnancy.

What Happens During an Ultrasound?

Ultrasounds are safe and painless procedures that use high-frequency sound waves to create images of your baby. The type of scan depends on your stage of pregnancy:

Abdominal Ultrasound

  • Probe is moved across the abdomen.
  • You may need a full bladder for better imaging.
  • A gel is applied to your belly to help the probe pick up sound waves.

Transvaginal Ultrasound

  • A thin probe is inserted into the vagina.
  • Used for early scans (before 8 weeks) or if clearer images are needed.

Why Might Your Due Date Change?

It's common for your due date to be adjusted after your ultrasound. Here's why:

  • Irregular Menstrual Cycles: If your periods aren't regular, your LMP-based due date might be inaccurate.
  • Late Ovulation: Ovulating later than day 14 of your cycle can shift the timeline.
  • Ultrasound Measurements: If the baby's size doesn't match the expected gestational age, your due date may be updated.

Comprehensive Comparison of Due Date Calculation Methods

Factor LMP-Based Due Date Ultrasound-Based Due Date
Method Counts 40 weeks from the first day of your last menstrual period (LMP). Uses imaging to measure the baby's size and estimate gestational age.
Information Needed The first day of your last menstrual period. Date of the ultrasound and measurements of the baby (e.g., crown-rump length).
Accuracy (First Trimester) Assumes a regular 28-day cycle with ovulation on day 14. Accurate if cycles are regular. Most accurate in early pregnancy (before 12 weeks).
Accuracy (Second Trimester) Less accurate if cycles are irregular or LMP is unknown. Reliable but less precise (+/- 1-2 weeks).
Accuracy (Third Trimester) Not used for precise dating as it becomes less reliable later in pregnancy. Least accurate due to varying growth rates of babies (+/- 2-3 weeks).
Changes Over Time Does not change once calculated. Can be adjusted based on ultrasound measurements if discrepancies arise.
When It's Used Used as the initial estimate if LMP is known and cycles are regular. Used as the primary method if LMP is unknown, irregular, or differs significantly.
Irregular Cycles Less reliable, as the ovulation date may vary significantly. More reliable, as it directly measures the baby's development.
When Performed Not performed – based on self-reported date of last period. Typically performed at 8–12 weeks (dating scan); may also occur later if needed.
Measurement Method Not applicable. Early pregnancy: Crown-rump length (CRL). Later pregnancy: Head, abdomen, and femur size.
Adjustment Rarely adjusted unless clear discrepancies arise. Adjusted if the ultrasound date differs by more than 5–7 days in the first trimester.
Purpose Provides an initial timeline for pregnancy tracking. Confirms gestational age, monitors growth, and provides a more accurate EDD.

Final Thoughts

Ultrasounds are a valuable tool for determining your due date and tracking your baby's development. While the earlier the scan, the more accurate the estimation, it's essential to remember that due dates are not fixed. Babies arrive on their own schedule, so be prepared for your little one to come anytime within the full-term range of 37 to 41 weeks.

If you have any concerns about your due date or pregnancy timeline, don't hesitate to consult your healthcare provider.