How often do I need to go in for prenatal testing?

Depends on risk. Prenatal care involves regular OB visits, but prenatal testing ranges from general screening (level I ultrasound, triple/quad/integrated screen, amniocentesis chromosomes) to gene tests when a prior child was affected--usually at 10-18 weeks. Realize that triple/quad/integrated screen looking for Down syndrome markers is inaccurate, sometimes leading to amnio and termination. Caveat emptor.