Amniocentesis done at:

Amniocentesis (genetic) is an invasive procedure. * Cytogenetic analysis--The desquamated fetal cells in the amniotic fluid, obtained by amniocentesis or trophoblast cells from CVS or fetal blood cells obtained by cordocentesis are cultured, G banded and examined to make a diagnosis of chromosomal anomalies, e.g. trisomy 21 (Down's syndrome),monosomy X (Turner's syndrome) and others. * DNA analysis--Single gene disorders (cystic fibrosis, Tay-Sachs disease) can be diagnosed using specific DNA probes. DNA amplification is done by polymerase chain reaction (PCR). The specific chromosomal region containing the mutated gene can be identified. * Biochemical--Amniotic fluid AFP level is high when the fetus suffers from open neural tube defects.This is also confirmed by ultrasound scanning. The normal AFP concentration in liquor amnii at the 16th week is about 20 mg/liter. Amniotic fluid level of 17 hydroxy progesterone is raised in congenital adrenal hyperplasia. Early amniocentesis has been carried out at 12-14 weeks gestation. Amnifiltration has been used to increase the cell yield. Reference: Dutta's Textbook of obstetrics 7th edition page no 106