Professor of Obstetrics and Gynecology
New York University School of Medicine
New York, NY
Wow, you certainly ask some important questions! Let's start with your question about oral contraception, or the birth control pill. Studies find that, used as recommended, oral contraception is 91 percent to 99 percent effective in preventing pregnancy. However, that still means there is a very small risk of pregnancy. In most instances, however, women who become pregnant while taking oral contraceptives either miss one or more doses; take a dose at a time different from their normal time (i.e., in the evening instead of the morning); take medications (such as antibiotics) that interfere with the way birth control pills work in your body; or have unprotected intercourse too soon after starting the pills.
This last reason answers your other question: If you begin taking birth control pills within six days of the first day of your period, it's effective immediately. If you start at any other time, however, you need to take it for a full month before it's effective, and you must use another form of birth control until you've completed one full cycle of pills.
You also ask about ejaculation and pregnancy. While ejaculation during intercourse is, obviously, the most common reason for pregnancy, you can get pregnant even if your partner ejaculates outside your vagina but close to the vaginal entry or withdraws his penis just before ejaculation. That's because sperm, by their very nature, are pretty hardy swimmers. And if you're ovulating and the sperm manage to make it from the vaginal entrance to your fallopian tubes, a pregnancy could certainly occur. Also, even though a man hasn't ejaculated during intercourse, some semen may still escape from his penis during intercourse. Keep in mind that it only takes one sperm and one egg—and one episode of sexual activity—to get pregnant.
If you absolutely do not want to get pregnant, yet you still plan to be sexually active, your partner should use a condom in addition to your chosen method of contraception. While no method of contraception other than sterilization is 100 percent effective against pregnancy, the more precautions you take, the less likely you are to get pregnant. Plus, if you are not in a long-term, monogamous relationship, your partner should be using a condom anyway to protect you both against sexually transmitted diseases.