Amniocentesis:
• Normally 15 to 20 weeks but in special circumstances may be done at any time after 15 weeks
• Collection of amniotic fluid transabdominally under ultrasound guidance
• Karyotype of fetal cells obtained from amniotic fluid – FISH or QF-PCR analysis results in 2-3 days, full karyotype from cell culture and microarray may be done
• Pregnancy loss rate traditionally quoted as 1/200; FASTER study data from USA suggests it could be much lower
• Failures may occur if inadequate sample (oligohydramnios), failure of fetal cell culture, or contamination of sample by maternal cells