What Is a Zygote?

Your pregnancy began with a zygote, which went on to form an embryo. Here's what to know about the early stage of human development.

Insemination, 3D Rendering

Getty Images

By now, you're probably familiar with the three main stages of pregnancy: the first, second and third trimesters. But do you wonder what your baby went through when it was nothing more than a single-celled zygote? Keep reading to learn more about zygotes, including what they are, how they form, and what happens to turn them into living, breathing babies.

What Is a Zygote?

When the egg and sperm come together (a process called fertilization), they form a zygote. This is essentially a fertilized egg, says Spencer Richlin, M.D., a board-certified fertility specialist in both Obstetrics and Gynecology and Reproductive Endocrinology and Infertility, and surgical director atIllume Fertility.

"At the time of ovulation, a mature egg is released from the ovary," explains Dr. Richlin. "This egg is then picked up by the fallopian tube. If no sperm is present, the egg will travel into the uterus and eventually be shed once a person's menstrual period begins. But if the egg joins with sperm and becomes fertilized, a zygote forms."

As it turns out, the zygote stage is over even faster than that precious newborn stage—your future little one will only be a zygote for a matter of days. But developmentally speaking, those were crucial days: even though it only contains a single cell, the zygote has all of the genetic material that will eventually turn into your baby.

How Are Zygotes Created?

People can either have intercourse or utilize assisted reproductive technology (such as intrauterine insemination and in vitro fertilization) to introduce sperm to the egg and achieve pregnancy, says Dr. Richlin. In either method, sperm will travel into the fallopian tube. "In the fallopian tube, the egg is surrounded by sperm. The best single sperm moves inside the egg and a zygote is formed," says Dr. Richlin. The zygote phase lasts for around four days; it eventually turns into a blastocyst, and then an embryo.

Zygote vs. Embryo: What's the Difference?

A chromosomally normal zygote has 23 pairs of chromosomes. At the time of fertilization, it has only one cell (made when the sperm cell combines with the egg cell). "The genetic information in these cells then causes the zygote to start dividing," says Dr. Richlin. Essentially, the single cell will become two cells, then four, then eight, and so on.

Next the zygote becomes a morula, which is a round structure of cells, and it travels down the fallopian tube, toward the uterus. The morula continues to divide and forms a blastocyst around the fifth day after fertilization. A blastocyst is a differentiated embryo with a fluid-filled cavity, containing future placenta cells and fetal cells.

The blastocyst implants into the uterine lining and a pregnancy begins. Your baby is now an embryo, and it will remain in the embryonic period until the eighth week of gestation—after that, it becomes a fetus.

How Does a Zygote Become a Twin?

Both types of twins—identical (monozygotic) and fraternal (dizygotic)—develop in the zygote phase. The major difference between them is in the number of eggs fertilized.

Identical Twins

Identical twins are formed when one egg is fertilized by one sperm in the fallopian tube, forming a single zygote. Before implantation, in the blastocyst phase, it splits into two pregnancies, says Dr. Richlin. "Identical twins are of the same sex with the same inherited characteristics." They're also called monozygotic twins.

Fraternal Twins

"Fraternal twins are formed when two eggs are fertilized by two separate sperm around the same time," says Dr. Richlin. This occurs when someone's body releases two eggs instead of one during ovulation. Because the eggs are fertilized by two separate sperm, fraternal pregnancies have two completely separate zygotes (which is why they're called dizygotic twins). Each zygote separately fertilizes, divides, grows, moves through the fallopian tube, and implants separately in the uterus as two embryos.

What Complications Can Happen in the Zygote Stage?

Complications can arise during the zygotic stage; this is actually one of the more common stages for problems to occur. In fact, Dr. Richlin points to research that indicates approximately 20% of pregnancies end "before pregnancy was detected clinically," which might be before implantation in the zygote stage.

Most complications at this point are related to chromosomal abnormalities, such as having an irregular number of chromosomes (which can lead to things like Trisomy 18, Trisomy 21, or Down Syndrome and Turner Syndrome), or having a chromosome with a structural abnormality. Many of these early losses occur before someone knows they're pregnant.

Was this page helpful?
Parents uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Incidence of Early Loss of Pregnancy. N Engl J Med. 1988.

Related Articles