Are first trimester ultrsounds accurate dating

Crown Rump Length: Gestational age = 6 weeks plus (CRL x days).For example, a CRL of 23 mm would correspond to a gestational age of 9 weeks and two days (6 weeks plus 23 days = 9 weeks and 2 days).This is normally less than 2.5mm and when seen increased (greater than 2.5mm, see image on the right) may indicate the baby has Down syndrome or may indicate another chromosomal abnormality. The nuchal translucency test will also check whether your baby has a visible nasal bone.In the past few years it has been seen that approximately 3 in 4 babies with Down's syndrome do not have a visible nasal bone at the time of the first trimester screening test.

The CRL (see image above between the two white marked 1) is highly reproducible and is the single most accurate measure of gestational age.

The 12 week scan is a routine ultrasound examination carried out at 10 to 14 weeks of gestation.

At the first trimester scan they confirm that the fetus is alive, they assess the gestational age by measuring the crown-rump length and will also look for any major problems.

Charts have been developed for this purpose, but some simple rules of thumb can also be effectively used.

From 6 to 11 weeks gestational age, the fetal CRL grows at a rate of about 1 mm per day.

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