Something kind of relevant I wrote for Absolute AvP 22 years ago.
"Which brings up an interesting and oft asked question, was Ripley pregnant following her sexual encounter with Clemens? And is this incarnation of Ripley pregnant also? There are a great many variables that need to be considered – Was Ripley on contraceptives? Was Clemens fertile (Morse mentions there's no rubbers, so that's out)? Does hypersleep screw up a woman's menstrual cycle? Does coming out of hypersleep the wrong way screw up a woman's menstrual cycle? Does carrying an Alien embryo screw up a woman's menstrual cycle? Was Ripley ovulating?
The minimum period between intercourse and conception is around 4-6 hours. There's a 14 hour gap between Ripley and Clemens having sex and Clemens giving Ripley his final needle, so there is a possibility, but when we take all the above variables into account that the fact it's never mentioned by the Doctors on the Auriga (and if anyone knew, they would), the chances of Ripley being pregnant are incredibly small."