12 Things You MUST Know About Birth Control Pills If You Don’t Want to Get Pregnant — or Do!

Updated: Mar. 17, 2022

Four of five sexually active women have used birth control pills. These are the facts their doctors hope they know.


How does birth control work?

There are basically two types of birth control pills, synthetic forms of the hormones progesterone and estrogen, which can be close to 100 percent effective at preventing pregnancy if used correctly. One type contains the female sex hormone progesterone and the other type contain a combination of estrogen and progesterone, explains Jill Rabin, MD, co-chief of the division of ambulatory care and Women’s Health Programs-PCAP Services at Northwell Health in New Hyde Park, New York. The hormones work in different ways to prevent pregnancy. “Progesterone thickens the cervical mucus, making it difficult for sperm to penetrate, and it also makes the fallopian tubes move slower so the egg and sperm don’t get together quite as efficiently,” she says. “It also makes the uterine lining hostile to implantation.” A fertilized egg can’t implant in a thin lining—it needs a soft cushy place to implant. Estrogen inhibits ovulation by suppressing the hormones responsible for ovulation. “If you interrupt this cycle, an egg won’t come out and a steady state of estrogen interrupts it,” Dr. Rabin says. Neither type of birth control pills protects against sexually transmitted diseases.


Are there any women who can’t take combined birth control pills?

“A lot of people can’t take estrogen,” says Dr. Rabin. The list includes women with a personal history of blood clots, breast cancer, or heart disease, and smokers. Progesterone alone is safer in some women. (These are the silent signs you’re on the wrong birth control.)


How long does it take for birth control to work?

“Some pills begin working within 72 hours and others may take three to five days to become effective,” says Dr. Rabin. “I tell my patients to use condoms for the first week just to be safe.” (These are the secrets your vagina wishes you knew.)


When should I take birth control pills?

This varies by type, Dr. Rabin says. “Progestin-only pills must be taken at the same time every day or your cervical mucus will start to thin and you could become pregnant,” she warns. “You don’t have to take combination contraceptives at the exact time every day, but doing so will make it become a habit.” Many women find it helpful to keep their pill pack next to their toothbrush, so the two daily habits naturally happen at the same time. Ask your doctor or pharmacist for advice on when and how you should take your pill.


What happens if I forget to take a pill?

“Take it as soon as you remember or take two the following day, and if you forget for two days in a row, it may be time to consider a long-acting form of contraception such as an IUD that doesn’t rely on memory,” says Dr. Rabin. And in the meantime, use condoms until you have another form of birth control in place.


How effective is the birth control pill?

“When we talk about the effectiveness of the pill, there are two different answers,” says Donnica Moore, MD, president of Sapphire Women’s Health in Chester, New Jersey. “In a clinical trial of when women use it 100 percent correctly as directed, the pill is more than 99.9 effective. However when it comes to real-life effectiveness, the pill is about 91 percent effective,” she says. “This means that 9 out of 100 pill users will get pregnant each year largely because it’s tricky to remember to take a pill every single day.”


Can anything interfere with the pill’s effectiveness?

Certain antibiotics, anti-fungal drugs, and other medications including some so-called natural supplements such as St. John’s wort can affect how well the pill works, Dr. Moore says. “Any time you get put on medication or start taking a new supplement and are also on birth control, ask your doctor or pharmacist if there are any interactions with the new medication and birth control pill and if so, what they are,” she says. If you have a stomach bug and have been vomiting or have diarrhea, you may not fully absorb your birth control pill. Back-up contraception may be required in these cases, according to Dr. Moore. (Here’s why you might be getting chronic diarrhea.)


Does the pill have any side effects?

“The major side effects from too much estrogen are bloating and sore breasts, so if you notice that, talk to your gynecologist about a lower-dose estrogen pill,” says Mary Jane Minkin, MD, clinical professor of Obstetrics & Gynecology at Yale University. “The major side effects from progestins (and there are a lot of different types out there) would be moodiness and irritability,” she says. “If that happens, do talk to your gynecologist about switching to a different pill that would make you a bit less moody.”


What should I do if I am on the pill and want to become pregnant?

Although you could just go off the pill, there are other considerations you’ll want to discuss with your doctor. For example, you might not know your blood sugar runs high, and according to Dr. Rabin, the time to fix this is before you become pregnant. “You should also be taking prenatal vitamins even when still on the pill to maximize your health,” she says. “Start prenatal vitamins, come off of the pill, see your doctor, and then get pregnant.” (Here’s how to get pregnant, according to OB-GYNs.)


How soon after stopping the pill can I try to conceive?

“Right away,” says Michael Cackovic, MD, an obstetrician/gynecologist at Ohio State University Wexner Medical Center in Columbus. “As a matter of fact, we sometimes put women who are irregular and want to conceive on the pill to regulate their menses.” The only way to know when you are ovulating is to get on a regular cycle, he adds. “Your fertility comes out kicking and screaming.”


Can I breastfeed while on the pill?

“There is a risk that women who go on the pill right after they deliver and want to breastfeed will have a diminished milk supply, but if you breastfeed for six weeks, your breast milk will be firmly established, so starting the pill should not affect your supply, ” says Dr. Cackovic.


What happens if I get pregnant while taking the pill?

This can and does happen, says Dr. Cackovic. “If you become pregnant while on the pill, stop taking it and have your obstetrician take a look at the baby at 18 weeks to make sure it’s fine,” he says. “If you continue taking the pill through your pregnancy, you will experience withdrawal bleeding each month.” If you suspect you are pregnant while on the pill, take a pregnancy test and make an appointment with your doctor.


With so many pills to choose from, how do I decide which one is right?

“I ask patients what they have been on before, and if it worked, we will go back on that one,” Dr. Cackovic says. Your doctor should also take a medical history to see if there are any contraindications to combination birth control pills. There are other considerations too, Dr. Rabin adds. Some birth control pills treat acne, too. And there’s a three-month pill where you take hormone pills every day for three months, followed by a week of placebo. Some women take active pills continuously for one year and never experience any menstrual bleeding. “There really is no medical reason that most women have to get their period every month.”