Midtrimester amniocentesis, the aspiration of amniotic fluid, has traditionally been performed between 15 and 18 weeks of gestation. Amniocentesis is routinely an outpatient facility and an ultrasound-guided procedure. Having confirmed fetal viability, an accessible pool of amniotic fluid should be determined. Ideally passage through the placenta should be avoided but, if this is not achievable, a transplacental approach may be employed. This chapter covers the in plane approach – needle beam alignment, multiple gestations, bloody tap, possible problems in amniocentesis, amniotic fluid sample collection and transport to the genetic lab, cytogenetic analysis, and safety.