Sex. Sexual intercourse is the method that our species - most mammals, actually - reproduce and create new human beings. It is quite effective.
Causes miscarriages
Unprotected sex causes pregnancy.
Nobody knows what causes women to smoke marijuana while they are pregnant.
People have kids through sexual intercourse.
== ==
Developing
Sperm fertilizing an egg.
Yes, it is very rare but can happen. The term for such a baby is a "mummified kit". Again, very rare and most often causes the doe to become infertile/sterile. Most people who "know" their rabbit is pregnant and then don't end up with a litter didn't have a pregnant doe. False pregnancies can also lead people to worry the litter is "stuck", but again, the doe wasn't actually pregnant.
No one should drink while they're pregnant. Drinking while pregnant causes fetal alcohol syndrome.
if you are pregnant it can cause you to miss some periods
No not everyone gets pregnant.
The only thing that causes a female to become pregnant is sperm coming in contact with her vaginal area.