CRL and Gestational Age Calculator
Estimate your baby’s exact Gestational Age (in weeks and days) and your Estimated Due Date (EDD) based on early ultrasound Crown-Rump Length.
Estimated Gestational Age:
What is Crown-Rump Length (CRL)?
During a first-trimester ultrasound, the sonographer will measure the length of the embryo or fetus from the top of its head (the crown) to the bottom of its buttocks (the rump). This measurement, known as the Crown-Rump Length (CRL), is usually recorded in millimeters (mm) and is the most accurate way to date a pregnancy and determine an estimated due date.
(Tracking your pregnancy? Use our Pregnancy Weight Gain Calculator to stay healthy! – Internal Link)
How is Gestational Age Calculated from CRL?
Because embryos grow at a highly predictable rate during the first trimester, the CRL measurement closely correlates to the baby’s age. The most standard medical formula (the Robinson rule) states:
Our calculator uses this established medical algorithm. Once the gestational age in days is calculated, it is converted into standard Weeks and Days format, and applied to the date of your ultrasound to accurately project your 40-week Estimated Due Date (EDD).
When is the CRL Measurement Most Accurate?
According to the American College of Obstetricians and Gynecologists (ACOG), ultrasound measurement of the embryo or fetus in the first trimester (up to 13 weeks and 6 days, or a CRL of 84 mm) is the most accurate method to establish or confirm gestational age. (Outbound Link)
After 14 weeks, the baby begins to curve and bend inside the womb, making a straight-line measurement impossible. At that point, doctors rely on other biometric measurements like Head Circumference (HC) and Femur Length (FL).
Frequently Asked Questions (FAQs)
1. My CRL dates differ from my LMP dates. Which is right?
If your CRL-calculated due date differs from your Last Menstrual Period (LMP) due date by more than 5-7 days (during the first trimester), medical professionals will generally change your official due date to match the ultrasound (CRL) date. Ovulation can be unpredictable, but early fetal growth is incredibly consistent.
2. Can CRL be used to determine the baby’s gender?
No. CRL is strictly a measurement of size and gestational age. While some early theories (like the “Nub Theory”) look at the angle of the genital tubercle around 12 weeks, the CRL measurement itself has nothing to do with the baby’s sex.
3. What if my CRL is measuring “small”?
If the CRL is slightly smaller than expected based on your LMP, it usually just means you ovulated later in your cycle than average. Your doctor will monitor your numbers to ensure steady, healthy growth at your follow-up appointments.
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