How does conception occur?

Conception (or fertilization) is when sperm and an egg join together. It’s one of the many steps that happen to create a pregnancy.

Conception is closely related to a person’s menstrual cycle. A menstrual cycle describes the sequence of events that occur within your body as it prepares for the possibility of pregnancy each month. Women or people assigned female at birth (AFAB) ovulate during their menstrual cycle. Ovulation is when your ovary releases an egg for fertilization. Tiny finger-like structures called fimbriae help guide the egg through your fallopian tubes towards your uterus. During this journey through your fallopian tubes, an egg can be fertilized by sperm.

Sperm production begins in the testicles of men or people assigned male at birth (AMAB). During ejaculation, millions of sperm cells are set free with the sole purpose of finding an egg to fertilize. When you have unprotected sex, sperm cells swim up through your vagina and into your fallopian tubes. Millions of sperm battle to reach and penetrate the egg, but only one breaks through the egg's outer layer to fertilize it. If sperm doesn't fertilize an egg, the egg dissolves.

If a sperm is successful on its quest to fertilize an egg, the now fertilized egg (called a zygote) continues to move down your fallopian tube, dividing into two cells, then four cells, then more cells. About a week after the sperm has fertilized the egg, the zygote has traveled to your uterus. It's now a growing cluster of about 100 cells called a blastocyst.

The blastocyst then attaches itself to the lining of your uterus (the endometrium). This attachment process is called implantation. However, just because conception occurs doesn't mean implantation will. Sometimes implantation doesn't happen, and you pass the fertilized egg in your next menstrual period.

If implantation happens, the cells continue to divide — some cells develop into your baby and others form the placenta. You begin to release hormones that tell your body a baby is growing inside your uterus. These hormones also signal the uterus to maintain its lining rather than shed it. This means you won't get your menstrual period, which may be the first way you know you’re pregnant.

Timeline of getting pregnant

You calculate your menstrual cycle from the first day of menstrual bleeding to the start of the next first day of menstrual bleeding. Most menstrual cycles are around 28 days long. The exact time you ovulate varies depending on how long your menstrual cycle is.

The process of getting pregnant in a 28-day menstrual cycle is:

  • Day one: First day of your period.
  • Around day 14: Ovulation occurs.
  • Within 24 hours of ovulation: Sperm fertilizes an egg (conception occurs).
  • About six days after fertilization: The fertilized egg implants into your uterine lining.
  • Around day 21: If conception and implantation occurred during this menstrual cycle, you're pregnant. However, getting a positive pregnancy test may take another five to seven days.

Conception and a positive pregnancy test

After conception, a fertilized egg travels through your fallopian tubes to your uterus. The fertilized egg (called an embryo) implants (attaches) into the wall of your uterus. This triggers the placenta to form. Your placenta begins producing and releasing human chorionic gonadotropin (hCG) into your blood and pee. HCG can be found in a person’s blood around 11 days after conception. It takes slightly longer for hCG to show up on at-home pregnancy tests (that measure hCG in pee).

What are my chances of conceiving?

Just because an egg and sperm are near each other doesn't mean conception will happen. In general, conception only happens 25% to 30% of the time. This percentage decreases once you reach age 35.

How does conception work with IVF?

Conception still works the same way — sperm must fertilize an egg. However, with in vitro fertilization (IVF), sperm fertilizes an egg in a lab. An egg, either from the intended parent or a donor, is mixed with sperm from a parent or donor. Conception happens when sperm fertilizes the egg.

Once conception occurs, your provider places the created embryo inside the uterus that will carry the pregnancy for implantation.

When does conception happen?

Conception occurs between 12 and 24 hours after ovulation. It’s sometimes hard to pinpoint ovulation, so using ovulation predictor kits or tracking your menstrual cycle on a calendar may be helpful. The two biggest factors in conception are:

  • The timing of sexual intercourse with ovulation.
  • Egg and sperm health.

When should I have sex to conceive?

Conception can happen after unprotected sex as early as five days before ovulation. This is because some sperm can live that long inside female reproductive organs.

If you’re trying to conceive, the best times to have sex are:

  • In the three days before ovulation: In this scenario, sperm will be “waiting” for the egg to come down the fallopian tube.
  • At ovulation or within 24 hours of ovulating: Your egg lives for only 24 hours, so if you have unprotected sex during this time, your egg may end up “waiting” for sperm to reach it, or they may run into each other in your fallopian tubes.

Where does conception happen?

Conception typically happens in your fallopian tubes. This is where an egg goes after it leaves your ovary and where sperm wait for an egg. In some cases, fertilization can happen in your uterus once your egg has left your fallopian tubes.

What things prevent conception from happening?

Certain health conditions may affect your ability to conceive. Just because the sperm and egg meet doesn't mean fertilization will occur. Some of the most common factors are:

  • Anovulation (you’re not ovulating).
  • Low sperm count or issues with sperm motility (how sperm move).
  • A blockage in the testicles, ovaries or fallopian tubes.
  • Decreasing amount of quality eggs and quality sperm (usually related to aging).

Can you feel conception?

Not usually. You may notice signs that you've ovulated, such as changes in your cervical mucus or basal body temperature. However, most people don't feel fertilization. You may feel a dull ache or experience light spotting several days after conception. This could be from the fertilized egg implanting in your uterus.

When do you start feeling pregnant?

How long it takes to “feel” pregnant varies. Some people may start to feel pregnant shortly after conception, while others don’t have any pregnancy symptoms for weeks after a positive test.

Common signs of pregnancy are:

  • A missed period.
  • Frequent urination.
  • Feeling tired.
  • Nausea.
  • Sore or swollen breasts.
  • Spotting (light vaginal bleeding).
  • Headaches.
  • Mood swings.

Take a home pregnancy test if you have any of the above symptoms and think there’s a chance you’re pregnant. Your healthcare provider can order a blood test to confirm pregnancy.

Are conception and fertilization the same?

Conception and fertilization are two different parts (or steps) of the same process. Conception is the first step, where an egg and sperm join. Fertilization is another step, where the joined sperm and egg plant like a seed into your uterine lining.

How long after conception will my pregnancy test be positive?

It can take between 11 and 14 days after conception to get a positive pregnancy test. At-home pregnancy tests check for human chorionic gonadotropin (hCG), a hormone released by the placenta. Your pee must have enough hCG to get a positive pregnancy test. However, your healthcare provider can check for hCG in your blood sooner — around 10 days after conception.

A note from Cleveland Clinic

Conception is when sperm fertilizes an egg. It’s one of the many critical steps in getting pregnant. Conceiving a child is a complex process dependent on lots of factors. Being unable to conceive is a common problem, and there are lots of resources available to help you. Contact your healthcare provider if you’re struggling with conceiving. They can explain the process and identify any issues preventing conception and pregnancy.

Last reviewed by a Cleveland Clinic medical professional on 09/06/2022.

References

  • March of Dimes. Getting pregnant. (https://www.marchofdimes.org/pregnancy/getting-pregnant.aspx) Accessed 9/6/2022.
  • Merck Manuals. Conception and Prenatal Development. (https://www.merckmanuals.com/professional/gynecology-and-obstetrics/approach-to-the-pregnant-woman-and-prenatal-care/conception-and-prenatal-development) Accessed 9/6/2022.
  • National Health Service. You and your pregnancy at 1 to 3 weeks. (https://www.nhs.uk/pregnancy/week-by-week/1-to-12/1-2-3-weeks/) Accessed 9/6/2022.
  • Office on Women's Health. Trying to conceive. (https://www.womenshealth.gov/pregnancy/you-get-pregnant/trying-conceive) Accessed 9/6/2022.

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