In That '90s Show, we learn (https://collider.com/that-90s-show-leia-real-dad-theory/) that Leia (https://that70sshow.fandom.com/wiki/Leia_Forman)'s 15th birthday is July 3, 1995. This means that she was born on July 3, 1980. But the last episode of of That '70s Show takes place on January 1, 1980 and Eric was in Africa prior to that. So Donna would have gotten pregnant somewhere in late September-early October 1979, while Eric was still in Africa.