Jessica Biel claims she didn't know what was going on inside her body before she got pregnant.

The 33-year-old actress, who has five-month-old son Silas Randall Timberlake with husband Justin Timberlake, admits she was worried about ''how hard'' it would be conceive her first child when she first decided to stop taking her birth control two years ago.

The 'Total Recall' actress said: ''[I thought] Now what happens? I've been on the pill for so long; how hard will it be to get pregnant?

''Suddenly I realised I really didn't know what's going on inside my own body. It was shocking.''

The brunette beauty has now launched a new online video series with activist Saundra Pelletier, who is also the founder of the non-profit health care organisation WomanCare Global, to ''share girl stories, fears, and insecurities'' about taboo subjects including puberty and contraception.

She told Glamour.com: ''You have to have the courage to stand up for what you believe in.

''We want girls to know what their [body is going through] so they don't feel scared or ashamed or gross. The tone is informative but also goofy, smart, witty.''

Saundra added: ''More than half of our nation's pregnancies are unplanned, and just 22 states require public schools to teach sex education. Jessica and I realised we can help change this.

''We need to get young adults this information in a variety of ways. There is power and dignity in understanding your body.''