During the show it was stated (and not disputed) that 1/3 of 20 year-old girls get pregnant.

[Statistics show that approximately four in ten girls are estimated to become pregnant before age 20.1] http://www.welfareacademy.org/conf/past/brown3.shtml#footnote1 

But according to:  http://www.nlm.nih.gov/medlineplus/teenagepregnancy.html#cat22

Results In 2002, an estimated 757,000 pregnancies among teenagers 15-19 years resulted in 425,000 live births, 215,000 induced abortions, and 117,000 fetal losses. The overall teenage pregnancy rate was estimated at 76.4 pregnancies per 1,000 females aged 15-19 years, down 10 percent from 2000 (84.8 per 1,000), and 35 percent lower than the peak rate in 1990 (116.8 per 1,000). The 2002 rate is an historic low for the Nation. Rates for young teenagers declined relatively more than for older teenagers throughout this 12-year period.

76.4 per 1000 = 7.6%

What's the truth? 7.6%, 33%, 40 %. ???