When people speak of conception they usually mean when a sperm fertilizes an egg. This is the first step in pregnancy. Fertilization can happen within hours or days of having sex without birth control.

Sperm can remain alive in the female reproductive tract for up to 5 days, but whether or not conception occurs will depend on various factors, including the stage of the menstrual cycle.

After fertilization, the egg descends into the uterus and attaches itself to the uterine lining. This is known as implantation. Both fertilization and implantation need to occur for someone to become pregnant.

Conception is not a medical term. Some people use it to refer only to fertilization, while others use it to refer to the overall process of creating a pregnancy. Because of this, scientists tend to use more precise terms.

This article discusses what conception is, when and how it happens, and how it works for those undergoing in vitro fertilization (IVF) or surrogacy.

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People usually define conception as the moment when a sperm fertilizes an egg. This can happen inside the body or outside of the body when people are using IVF.

However, conception is not a scientific term. It comes from social and philosophical ideas about when human life begins. Some doctors use this word, but others may prefer “fertilization”, which is more specific.

Conception does not always lead to pregnancy. A person is technically not pregnant until implantation has occurred. This is the moment when the fertilized egg implants into the lining of a uterus, which occurs around 5–6 days after fertilization.

Not all fertilized eggs get to this stage. An estimated 50% of all fertilized eggs do not implant and leave the body the same way as unfertilized eggs do — during menstruation.

Conception can happen just after ovulation. Ovulation is the point in the menstrual cycle when the ovaries release an egg.

In a regular cycle, ovulation usually occurs around 10–16 days before a person’s next period. However, many have irregular cycles that change in length every month, so the exact day of ovulation often varies.

Fertilization can then happen in several ways:

  • Sexual intercourse: During sex, sperm can enter the vagina. From here, it can swim up toward the uterus and find its way to the egg. This can take several days, with healthy sperm surviving inside the body for up to 5 days.
  • Intrauterine insemination (IUI): This involves a doctor inserting sperm into the uterus through a thin tube. A doctor may do this for people trying to get pregnant via sperm donation or for those opting for surrogacy. Surrogacy is when a person other than the biological parents carries the pregnancy.
  • IVF: This involves a doctor taking sperm and eggs from two people and combining them in a laboratory setting. IVF cannot guarantee fertilization will occur but creates the best circumstances possible for it to happen.

Conception in IVF is unique, as it takes place outside of the human body. However, it is still conception.

The process typically involves:

  • a female taking a course of fertility drugs, which cause the ovaries to release more eggs than usual, and at predictable times
  • a doctor removing these eggs from the body after ovulation
  • a doctor fertilizing the eggs with sperm, either from a donor or a partner
  • the fertilized eggs growing outside of the body for several days until they become embryos
  • a doctor inserting the embryos into the uterus

If at least one embryo implants, then pregnancy begins.

With surrogacy, conception can occur inside or outside the surrogate’s body. It can depend on whether the surrogate’s eggs are involved, or whether they are receiving eggs from someone else.

If they are using their own eggs, a doctor may use IUI to create the circumstances for fertilization inside the body. If the surrogate is not using their own eggs, a doctor will use IVF to fertilize eggs outside the body.

Learn more about IVF here.

After conception, a fertilized egg – or zygote – may or may not implant. If it does implant, pregnancy has begun.

At this point, the zygote cells begin dividing, and the zygote becomes an embryo. This the scientific name for an organism with multiple cells. After 8 weeks, the embryo becomes a fetus.

Learn more about pregnancy here.

How to tell pregnancy has begun

The only reliable way to know if pregnancy has begun is to take a pregnancy test. This will only work after implantation.

This is because pregnancy tests detect human chorionic gonadotropin (HCG), an important pregnancy hormone. The body starts producing this in higher quantities after implantation. Steady increases in HCG are a sign of early pregnancy.

Pregnancy loss

While many pregnancies continue to full term, not all do. Around 26% of pregnancies end in miscarriage. A miscarriage is when the pregnancy spontaneously ends before 20 weeks.

Learn more about miscarriage here.

Conception is only possible when sperm have access to an egg. Anything that interferes with this may prevent conception. This could include:

  • a low sperm count, which means there is a low volume of sperm in a male’s ejaculate
  • sperm that are not motile, meaning they do not move or swim efficiently
  • anovulation, which is when someone does not ovulate
  • infrequent or irregular ovulation, which can make conception more difficult
  • inflammation or damage to the ovaries or uterus
  • blockages in the fallopian tubes

In IVF, conception can be prevented if fertilization does not occur successfully or if the embryos stop growing before a doctor implants them. Embryo quality can also be low, meaning there is a low chance of an embryo implanting into the lining of the uterus.

People can intentionally prevent conception via contraceptives, which are a group of devices and medications. They include:

  • condoms
  • diaphragms
  • intrauterine devices (IUDs)
  • hormone pills, patches, injections, or implants

Learn about the different types of contraception here.

People often use the term conception to refer to fertilization, when sperm fertilizes an egg. It can also refer to the overall process of becoming pregnant, which includes fertilization and implantation. A person is not pregnant until a fertilized egg, or zygote, implants in the uterine lining.

Conception can happen in several ways, both with or without the help of fertility treatments and reproductive technology.