Ultrasound in Pregnancy

Trimester Scans

• Early First Trimester Scan
• Late First Trimester Scan
• The Second Trimester Scan

Will the scan reveal the sex of my baby?

Sex of the baby can be interpreted from 18-20 weeks depending on the position of the baby. It becomes almost impossible in a case of complete breech. India has passed a rule that it is illegal to reveal the sex of the child in view of increasing incidence of female feticides.

Assessment of growth by ultrasound

The assessment of gestational age is important for evaluation of growth restriction.

• Crown rump measurement between 6-12 weeks provides gestational age with an error of only 3 days.
• Early second trimester ultrasound scans up to 20 weeks show a 1 week variation.

Ultrasound accuracy is inversely proportional to the fetal age.

• The parameters used in the third trimester include biparietal diameter, head perimeter, occipito-frontal distance, cephalic index, femur length, diameter of distal femoral epiphysis. More the parameters considered more the accuracy.
• Fetal cranial measurements are the most sensitive indicators for assessment of fetal growth.
• Growth restriction if any is classified as symmetrical (if all the parameters and head and chest growth is compromised) or asymmetrical (if the head and the chest parameters are not compromised to the same degree.

Amniotic Fluid Assessment

In the second and the third trimester, amniotic fluid volume is largely contributed by fetal urinary production.

To assess this by ultrasound, amniotic fluid index is used.

This index is the sum of maximum vertical depth of the deepest pocket free of fetal parts and cord, in each of four uterine quadrants.

Amniotic fluid index of

90-120mm              represents a mild oligohydramnios

60-90mm              moderate oligohydramnios

Less than 60mm  severe/critical oligohydramnios

The normal fluid volume is also an indirect way to prove a normal growth of the fetus.