Are first trimester ultrsounds accurate dating
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.
The top line represents the skin and bottom one, represents the nasal bone.
A third line, almost in continuity with the skin, but at a higher level, represents the tip of the nose.
The fluid collection is however increased in many fetuses with Down's syndrome and many other chromosomal abnormalities.
It is called a 'translucency' because on ultrasound this appears as a black space beneath the fetal skin.
The baby will be measured, and the anatomy examined in detail.
It is recommended that all pregnant women have a scan in the first trimester at around 12 weeks of pregnancy to confirm their dates.
Otherwise a scan is generally booked in the first trimester to confirm pregnancy, exclude ectopic or molar pregnancies, confirm cardiac pulsation and measure the crown to rump length for dating.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 crown rump length is a universally recognized term, very useful for measuring early pregnancies.The CRL (see image above between the two white marked 1) is highly reproducible and is the single most accurate measure of gestational age.