If my partner ejaculates in the shower while im in there, and the drain gets clogged, is there any possibility i could get pregnant from the back spray from the shower head?

Not in a million years. Pregnancy is possible with actual intercourse. If you and your partner are considering actual intercourse, use appropriate contraception to avoid pregnancy.

Pregnancy is not possible in above circumstances. Contraception remains otherwise key in avoiding undesired pregnancy and associated anxiety and when intercourse is likely to occur.