Technically only impregnation is implemented, since you can choose to cum inside or not and if you do choose to cum inside, then the girl could possibly get pregnant, chances increasing each each date and each time you do that.
But, the girls getting pregnant hasn't been implemented yet, so only technical impregnation so far.
Cumming inside girls is a good start. However I find that the missing part here is the ability to know when a girl is fertile or on the pill. When the girl is fertile or ovulating, the dialogues (conversations) when having sex should be naughtier, mentioning about risk of being pregnant. I think that the game should have 3 kinds of sex scenes in this case:
+ Submissive screens:
the son controls everything and talks openly about impregnating or inseminating. There should be options to allow girls to take pill before or after the intercourse or even exchanging the real pill with fake one, making girls think that she is protected but actually not. The girl can be braver when talking about pregnancy for pure fantasy because she feels safe but actually she isn't. Dirty talk is fun.
+ Romantic screens:
where the mother/sister allows the son to breed her (not just having sex). The game Incest Story 2 has some such screens with sister and mother who get the son's sperm deep inside their womb, thinking that they can be pregnant. Actually, there are 3 ending screens where the son impregnated his mother, sister and aunt. Deep Impact has such screens too in 0.05 (last version)
In romantic screens, I think that it is very nice to have options to talk about pregnancy that openly or keep in mind. It is really nice to read mind of the both side about risk of pregnancy.
When the son tries to impregnate a girl, he even doesn't let the cum to pour out. An internal view into creampied pussy/womb would be perfect. The game Deep Impact has a X-Ray animated screen to show how cum is deposited into womb. It is fantastic.
Romantic screens come with romantic events where the son must do something great to make girls extremely happy and feel safe.
E.x 1: sad and lonely birthday party reminds the mother about her memory when she met the father, she was bred and pregnant in the day. She wants to be loved and bred again by someone who really loves her and protect her.
E.x 2: the son saves the mother/sister from being violated (or did something great) and is rewarded 2 days of sex of his choice. If the son is corrupted he can choose the day the girl is fertile or hide the pill or exchange the pill with fake one. Or he can openly request breeding (fertile day, unprotected sex, creampie, no pill so purely animal breeding). The son's urge to inseminate his mother/sister eggs can be sources of dirty talks and thoughts.
I find that literotica.com or asstr.org are very good sources for family sex and impregnation.
+ Forced sex: I think that many many guys here like forced sex at some points of time. It is purely fantasy. The son will never accept the girl requests to pull out. He forces his virile sperms into the mother/sister/girls's fertile womb. He even waits for girls to being fertile/ovulating to breed them. The Tyrant is full of forced sex.
My suggestion is about impregnation, not pregnancy. I think that pregnancy can appear in ending screens like in The Incest Story 2 with multiple position, multiple creampie, intense breeding. Such breeding scenes should be romantic. The girl agrees to be bred and has a baby. Who suggests breeding and having a baby should be an option that user can select.