Wasn't there some mention of that drug Sterling had taken affecting his sperm, meaning it could possibly bypass contraception? If that's the case, it's open season for any of the girls to get pregnant.
EDIT:
They did. I guess it depends on what type of contraception the others are using:
"'Do not use if you or your partner are pregnant, plan to become pregnant, or while breastfeeding.'"
"'Sugar Cum is not to be used if you or your partner use a copper intrauterine device.'"
"'Use of Sugar Cum can result in the increase of cystein rich proteins called Metallothioneins that bind to copper. Use appropriate precautions."