Hayek, a paid spokeswoman for Pampers' tetanus vaccine program, recounted her experiences during a recent trip to the West African nation of Sierra Leone, where she met with tetanus victims.
"One of the things that was very moving about the trip was to see 15-year-old girls, really young, taking responsibility for their lives and their children before they're born by saying 'I am going to be healthy, I am going to take this vaccination,"' she told journalists at the United Nations on Thursday.
"I had no idea how much this was going to really personally move me," added the 42-year-old star of films including "Desperado" and "Frida."
The Pampers-UNICEF partnership has already provided over 50 million vaccines to mothers and babies in developing countries, where tetanus kills up to 140,000 infants and 30,000 women each year, according to the U.N. agency.
Pampers, owned by Procter & Gamble Co., said it would give UNICEF the money for one tetanus vaccine for every pack of specially marked diapers it sells before the end of the year. It expects that to produce 70 million more vaccinations.
Hayek's daughter Valentina Paloma Pinault was born in September 2007.