Fertilization occurs in the fallopian tubes. The fertilized egg may then implant on the wall of the uterus, leading to pregnancy.

There are many misconceptions about fertilization and pregnancy. Many people don’t understand how and where fertilization takes place, or what happens as an embryo develops.

While fertilization can seem like a complicated process, understanding it can equip you with knowledge about your own reproductive system and empower you to make decisions.

Let’s take a closer look at 10 facts about fertilization. Some of these may even surprise you.

Many people think fertilization occurs in the uterus or ovaries, but this isn’t true. Fertilization takes place in the fallopian tubes, which connect the ovaries to the uterus.

Fertilization happens when a sperm cell successfully meets an egg cell in the fallopian tube. Once fertilization takes place, this newly fertilized cell is called a zygote. From here, the zygote will move down the fallopian tube and into the uterus.

The zygote then burrows into the uterus lining. This is called implantation. When the zygote implants, it’s called a blastocyst. The uterus lining “feeds” the blastocyst, which eventually grows into a fetus.

An exception to this rule would happen with in vitro fertilization (IVF). In this case, eggs are fertilized in a lab.

If your fallopian tubes are blocked or missing, it’s still possible to get pregnant via IVF, as fertilization will take place outside your body. Once an embryo is fertilized using this method, it’s transferred to the uterus.

Ovulation is when a mature egg is released from one of your ovaries. If you ovulate and a sperm cell doesn’t successfully fertilize the egg, the egg will simply move down the fallopian tube, through the uterus, and out through the vagina. You’ll menstruate about two weeks later when the uterus lining is shed.

There are a number of reasons why fertilization might not happen. This includes use of contraception and infertility. If you’re having difficulty getting pregnant and have been trying for over a year (or more than six months if over the age of 35), speak to your healthcare provider.

Usually, only one egg is released during ovulation. However, the ovaries sometimes release two eggs at once. It’s possible for both eggs to be fertilized by two different sperm cells. In this case, you might become pregnant with twins.

These twins will be known as fraternal twins (also called nonidentical twins). Because they come from two separate egg cells and two separate sperm cells, they won’t have the same DNA and might not look identical.

Fertility treatments like IVF can increase the likelihood of multiple births, according to Cleveland Clinic. This is because fertility treatments often involve transferring more than one embryo to the uterus at a time to increase the chances of pregnancy. Fertility drugs can also result in more than one egg being released during ovulation.

Sometimes, a single embryo splits after it’s been fertilized, resulting in identical twins. Because both cells come from the exact same egg cell and sperm cell, identical twins will have the same DNA, the same sex, and a nearly identical appearance.

At the point of ovulation, the uterus wall is thick. Barring any complications, the fertilized egg (embryo) should go on to implant in the uterus by “sticking” to the thickened uterus wall.

The American College of Obstetrics and Gynecology (ACOG) considers someone pregnant only once the embryo is successfully implanted against the uterine wall. In other words, implantation marks the beginning of a pregnancy.

The embryo, however, might not implant. Emergency contraception, intrauterine devices (IUDs), and infertility could prevent the embryo from implanting.

Standard oral contraception and emergency contraception pills (“Plan B”) prevent ovulation. In the event that ovulation has already occurred when you take Plan B, the U.S. Food and Drug Administration notes that it may prevent a fertilized egg from implanting.

An IUD works by thickening cervical mucus. This can both prevent ovulation and create an environment that kills or immobilizes sperm, preventing the possibility of fertilization.

Since you’re only considered pregnant by ACOG once implantation happens, IUDs don’t end a pregnancy. Rather, they prevent pregnancy from happening. ACOG notes that IUDs and emergency contraception aren’t forms of abortion, but contraception.

IUDs and emergency contraceptive pills are both extremely effective forms of contraception. According to the World Health Organization, both are 99 percent effective at avoiding pregnancy.

If the fertilized egg burrows somewhere else other than the uterine lining, it’s called an ectopic pregnancy. About 90 percent of ectopic pregnancies occur when the embryo implants in one of the fallopian tubes. It could also attach to the cervix or abdominal cavity.

Ectopic pregnancies are medical emergencies that need prompt treatment to prevent a tube rupture.

After implantation occurs, the placenta forms. At this point, your body will produce the hormone human chorionic gonadotropin (hCG). According to Mayo Clinic, hCG levels should double every two to three days in the early stages of pregnancy.

Pregnancy tests work by detecting hCG in your body. You can either test your urine, as with home pregnancy tests, or test your blood via your healthcare provider. If you’re testing your urine with a home pregnancy test, do the test first thing in the morning, as that is when your urine is the most concentrated. This will make it easier for the test to measure your hCG levels.

The “gestational age” of a pregnancy is the duration of the pregnancy. When you find out you’re pregnant, your doctor or midwife might count the gestational age of your pregnancy in increments of weeks. Most babies are born in week 39 or 40.

Many people think that the gestational age begins at fertilization, with “week 1” being the week you got pregnant, but this isn’t the case. Week 1 is actually retroactively counted from the first day of your last period. Since ovulation usually occurs around 14 days after the first day of your period, fertilization usually takes place in “week 3” of pregnancy.

So, for the first two weeks of the gestational period, you’re not actually pregnant at all.

The difference between an embryo and a fetus is gestational age. Until the end of week 8 of pregnancy, the fertilized egg is called an embryo. In medical terms, it’s considered a fetus from the beginning of week 9 onwards.

At this point, all major organs have begun to develop, and the placenta is taking over many of the processes such as hormone production.

Whether you’re trying to get pregnant or curious about the science behind pregnancy, it’s important to learn about the fertilization process. Knowing about reproduction can help you get pregnant, make better decisions about contraception, and understand your own body better.