Where Does Fertilization Occur? 10 Facts That May Surprise You

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HealthlineSubscribeParenthood10 Things to Know About Fertilization

Medically reviewed by Stacy A. Henigsman, DO — Written by Sian Ferguson — Updated on June 20, 2025

Where fertilization occurs
When fertilization occurs
Fraternal twins
Identical twins
Implantation
Emergency contraception and IUDs
Ectopic pregnancy
HCG levels
First week of pregnancy
Embryo vs. fetus
Takeaway

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. 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.

How and where does fertilization take place? What happens as an embryo develops? Let’s take a closer look at 10 facts about fertilization. Some may even surprise you.

Fertilization occurs in the fallopian tubes

Many people think fertilization occurs in the uterus or ovaries, but fertilization actually 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 uterine 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.

»MORE:Can ovarian cancer start in the fallopian tubes?

Fertilization doesn’t always occur, even if you ovulate

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 2 weeks later when the uterine lining is shed during your period.

There are a number of reasons why fertilization might not happen, including infertility. If you’re having difficulty getting pregnant and have been trying for over a year (or more than 6 months if over the age of 35), speak to your healthcare professional.

»MORE:Can stomach ulcers cause infertility?

A fraternal twin pregnancy occurs when two eggs are released during ovulation, and both eggs are fertilized

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 (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. 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.

IVF twin pregnancies are also often more clinically challenging.

An identical twin pregnancy occurs when the fertilized egg splits

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.

»MORE:do identical twins have the same fingerprints?

The fertilized egg implants in the uterus

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

When an embryo is successfully implanted against the uterine wall, it starts releasing pregnancy hormones. In other words, implantation marks the beginning of a pregnancy.

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

»MORE:Natural ways to boost fertility

Emergency contraception pills and IUDs are not forms of abortion

Standard oral contraception and emergency contraception pills (“Plan B”) prevent ovulation. Plan B will not abort an existing pregnancy.

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 pregnant once implantation happens, IUDs don’t end a pregnancy. IUDs and emergency contraceptive pills are both extremely effective forms of contraception. According to the World Health Organization (WHO), both are 99% effective at avoiding pregnancy.

An ectopic pregnancy is when a fertilized egg implants outside the uterus

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

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

»MORE:What to expect durng an ultrasound for ectopic pregnancy

Pregnancy tests detect hCG in your urine or blood

After implantation occurs, the placenta forms. At this point, your body will produce the hormone human chorionic gonadotropin (hCG). Levels of hCG typically double every 24 hours during the first 8 weeks 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 professional.

If you’re testing your urine with a home pregnancy test, do the test first thing in the morning, as that’s when your urine is the most concentrated. This will make it easier for the test to measure your hCG levels.

»MORE:The best pregnancy tests

Week 1 of your pregnancy is counted from the first day of your last period, not from fertilization

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.

It’s a misconception 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 2 weeks of the gestational period, you’re not actually pregnant at all.

From week 9 of pregnancy, the embryo is considered a fetus

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 onward.

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

The takeaway

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.

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How we reviewed this article:

SourcesHistoryHealthline has strict sourcing guidelines and relies on peer-reviewed studies, academic research institutions, and medical journals and associations. We only use quality, credible sources to ensure content accuracy and integrity. You can learn more about how we ensure our content is accurate and current by reading our editorial policy.

Betz D, et al. (2025). Human chorionic gonadotropin. (2025).https://www.ncbi.nlm.nih.gov/books/NBK532950/
Ectopic pregnancy. (2018).https://www.acog.org/Patients/FAQs/Ectopic-Pregnancy
Emergency contraception. (2018).https://www.acog.org/Patients/FAQs/Emergency-Contraception
Family planning/contraception. (2018).http://www.who.int/news-room/fact-sheets/detail/family-planning-contraception
Guo C, et al. (2025). The clinical characteristics differences between IVF twins and naturally conceived twins with preterm infants.https://pmc.ncbi.nlm.nih.gov/articles/PMC11936628/
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How pregnancy happens. (n.d.).https://www.plannedparenthood.org/learn/pregnancy/how-pregnancy-happens
Identical twins. (2025).https://www.genome.gov/genetics-glossary/identical-twins
Plan B One-Step (1.5 mg levonorgestrel) information. (2022).https://www.fda.gov/drugs/postmarket-drug-safety-information-patients-and-providers/plan-b-one-step-15-mg-levonorgestrel-information
Reed BG, et al. (2018). The normal menstrual cycle and the control of ovulation.https://www.ncbi.nlm.nih.gov/books/NBK279054/

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Medically reviewed by Stacy A. Henigsman, DO — Written by Sian Ferguson — Updated on June 20, 2025

Tag » Where Does Fertilization Occur In Human Body

Where Does Fertilization Occur? 10 Facts That May Surprise You

Fertilization occurs in the fallopian tubes

Fertilization doesn’t always occur, even if you ovulate

A fraternal twin pregnancy occurs when two eggs are released during ovulation, and both eggs are fertilized

An identical twin pregnancy occurs when the fertilized egg splits

The fertilized egg implants in the uterus

Emergency contraception pills and IUDs are not forms of abortion

An ectopic pregnancy is when a fertilized egg implants outside the uterus

Pregnancy tests detect hCG in your urine or blood

Week 1 of your pregnancy is counted from the first day of your last period, not from fertilization

From week 9 of pregnancy, the embryo is considered a fetus

The takeaway

How we reviewed this article:

How Pregnancy (Conception) Occurs - MyHealth Alberta

Where Does The Fertilization Occur In Human Body? - Byju's

How Fertilization Happens - Getting Pregnant - What To Expect

Human Fertilization - Wikipedia

Where Do Human Sperm And Egg Meet? - PubMed

How Does Fertilization Occur In Humans? - Mannat Fertility Clinic

How Does Fertilization Take Place In Human Beings?

Conception: Fertilization, Process & When It Happens

What Are The Steps Of Fertilization In Humans? - InviTRA

Where Does Fertilization Occur? - CoFertility

Fertilization Occurs In Human Beings In - Toppr

How Fertilization Occurs In Humans - VittoriaVita

Fertilization And Implantation - Mayo Clinic

Conception: How It Works | Patient Education - UCSF Health

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