What's the difference between a pregnant lady and a lightbulb? You can unscrew a lightbulb
What is it about lightbulbs and pregnant women?
Both were screwed without their consent
While some pregnancies are unexpected, some people plan them and sometimes need medical intervention to get pregnant. Either way joking about unexpected pregnancies can make people feel like their children are not wanted which could make the child feel bad. You would feel upset if you thought people didn't want you around.