A 35 year old female, mother of a Down's syndrome child, became pregnant. Amniocentesis is done to rule out Down's syndrome in the second child. What is the appropriate time for doing amniocentesis?

Amniocentesis generally is carried out at 15 to 17 weeks of gestation but can be offered earlier (12-14 weeks). The underlying risk of amniocentesis when performed at 15 weeks of gestation and beyond is increased risk of miscarriage. This risk is estimated at 1 in 200 (0.5%). Ref: Manipalviratn S., Trivax B., Huang A. (2007). Chapter 5. Genetic Disorders & Sex Chromosome Abnormalities. In A.H. DeCherney, L. Nathan (Eds),CURRENT Diagnosis & Treatment Obstetrics & Gynecology, 10e.