A positive pregnancy test is always a heart-pounding moment with mixed emotions and anticipation. You wait for the following doctor visit eagerly in anticipation of knowing your gestational age.

Crown-rump-length – CRL in pregnancy ultrasound scan is a critical measurement that helps your doctor to calculate the gestational age of your baby (age of pregnancy).

What is CRL in Pregnancy?

A Crown-Rump-Length (CRL) evaluation is done during your pregnancy scans to estimate the gestational age of your baby by measuring the distance from the top of the head (crown) to the bottom of the rump (buttocks).

Baby in ultrasound scan
An ultrasound of a human fetus measured to be 1.67 cm from crown to rump and estimated to have a gestational age of 8 weeks and 1 day. Image courtesy: Wikipedia

How is CRL measured in pregnancy scans?

CRL measurement in pregnancy is estimated during the first trimester and is done between six and fourteen weeks approximately.

The baby’s height is measured from the top of its head to the bottom of the buttocks by excluding the limbs and yolk sac measurement.

The accuracy of CRL measurement in determining the gestational age is within 3 to 5days. 7 to 9 weeks of gestation is the optimum window for the CRL measurement using a transvaginal ultrasound scan.

The reliability of the estimated measurement at 8weeks is ±5days and at 12 weeks is ±1 weeks.

How is a baby’s growth measured after 14weeks?

Once the baby has passed fourteen weeks of gestational age, the head circumference, biparietal diameter, abdominal circumference, and femur length are used to calculate the baby’s development. These measurements are studied accurately in a 28 weeks growth scan. 

Why is CRL measurement important in pregnancy?

1. Gestational age of Your Baby:

Gestational age is a common medical term used to indicate pregnancy age. In short, this is the length of time a fetus has been growing inside the mother’s uterus.

Your doctor will compare the CRL measurement to your date of last menstrual period (LMP) to calculate your baby’s gestational age.
This data is more helpful in finding gestational age in case of irregular menstrual history.

2. Predictor of Missed Miscarriage:

Once the CRL measurement exceeds 7mm, your doctor should be able to detect the cardiac activity of your baby. If this is not detected, there is a possible chance of missed miscarriage.

This means the baby has died in the womb, but the mother didn’t show any normal miscarriage symptoms like bleeding or pain.

3. Predictor of Low birth weight:

A study on 316 transvaginal ultrasound scans revealed a positive correlation between CRL and birth weight when CRL was measured at days 46, 53, 60, 67, and 74 of gestation.

Further studies on this concluded that among all perinatal (the period of time when you become pregnant and up to a year after giving birth) complications, only SGA has a significant relationship with CRL discordance
>11%.

Normal Value Of CRL In Pregnancy?

Approximate fetus CRL measurement at different stages of gestation

Factors that Affect the CRL Measurement

  • Mother’s age
  • Amount of folic acid consumed
  • Smoking habit of a mother
  • Variation with different babies
  • Benchmark measurements of your doctor
  • Chromosomal disorders

CRL Measurement in Multiple Pregnancy

With multiple pregnancies, CRLs correlate between co-twins. But there are possibilities of variabilities to be observed in multifetal pregnancies. 

Estimation of gestational age in multiple pregnancies after the second trimester is not advisable since growth curves in multiple pregnancies vary from those in singleton pregnancies beyond that.

What does small CRL mean?

Decreased CRL can predict chromosomal disorders like Down syndrome, Edward’s syndrome, Patau syndrome, or any other abnormalities associated with growth restriction.

The first-trimester pregnancy with a small gestation and a decreased CRL measurement has an increased likelihood of miscarriage.

What does large CRL mean?

Babies with LGA (larger than gestational age) are characterized by CRL measurements larger than expected at 11 to 14 weeks gestation.

Significance of MSD and CRL measurements

Mean sac diameter (MSD) measures your gestational sac, which I usually see in a scan after 3 weeks of conception. It measures about 2 to 3mm by 5 weeks after conception.

MSD measurement is equally important as the measurement of CRL in pregnancy.

For a healthy gestation, MSD is expected to be at least 5mm greater than your baby’s crown-rump length (CRL).

If your MSD is less than 5 mm greater than the CRL measurement, a first-trimester miscarriage may be imminent. This can occur despite a normal heartbeat in your baby.

Closure Note:

In pregnancies destined to result in a fetal loss, various ultrasound markers become abnormal atleast a week before the loss.

It is not necessary that these soft markers should be seen only during the first few weeks of gestation. Fetal demise and abnormalities can show up at different stages of pregnancy.

This is why the recent scientific protocols stress the importance of critical ultrasound markers to be studied in all the ultrasound scans taken throughout the pregnancy.

Every scan and pregnancy check-up advised by a doctor is essential, and parents should make sure they do not miss any of them.
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