Lately I have been getting inundated with questions from women ending with "Am I pregnant?" so I just want to clear up what needs to happen in order for you to even think there could be a remote possibility that you are preggers.
In order to get pregnant, you must have sex when you are ovulating, about 10-18 days after the first day of your period. During ovulation, your body releases an egg from one of its ovaries (you have two) each month and in order to get pregnant, a sperm has to unite with your egg. Since sperm can live up to 5 days inside a woman's body, technically a sperm could still be swimming inside you in which case it is possible it can fertilize your egg. Stress and taking certain meds can make you ovulate later in your cycle or not at all, which is why your period can be "late," so if you are worried you might be pregnant, take a pregnancy test.
What about if a guy ejaculates outside of your body? To find out,
If a guy ejaculates outside of your body, you can't get pregnant unless his semen comes in contact with your vagina. After he ejaculates, if you're still wanting to continue your intimacy, it's a good idea for both of you to wash your hands and any area where his sperm is present just in case.
Also, having sex while you have your period is usually a "safe" time, but if you don't want to end up a mom, use birth control! Find a method that's right for you. Condoms work well, but the guy MUST put one on before having sex, even if it's for a few minutes. Why? Pre-cum is a liquid that comes out of a man's penis before he ejaculates and it can contain sperm, so you don't want that inside you whatsoever since pre-cum can get you pregnant.
The Pill, the NuvaRing, and other hormonal birth control methods are great too. They prevent you from ovulating in the first place so they are very effective at protecting you against pregnancy, but only when used properly. They don't however protect you from STIs - only condoms can do that.
This can be a confusing subject, so feel free to keep the questions coming. It's better to know the facts than to end up pregnant.