It depends on the individual, but the short answer is that if you and your partner have hetero (one male, one female) reproductive systems, you’re at risk for a pregnancy. So if you’re a transwoman with a penis and your partner has a female reproductive system, your partner should definitely use birth control. Same if you’re a transman with a female reproductive system—if you ever have vaginal sex with a partner who has a male reproductive system, you should use birth control. And remember, most birth control methods don’t protect from STIs—so use condoms and dental dams for that!