What Is a Zygote?

The Single-Cell Stage After the Egg Is Fertilized

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A zygote, also called a fertilized egg, is the phase of conception where the egg and sperm join to form a single cell. The zygote contains a full set of chromosomes, with 23 from the egg and 23 from the sperm. The zygote phase lasts only about four days, after which the single cell splits rapidly to become a blastocyst and then an embryo.

3d illustration of transparent sperm cells swimming toward egg cell

Christoph Burgstedt / Getty Images

Formation

A zygote forms when a sperm penetrates the outer surface of an egg. This happens in the fallopian tube. While the zygote stage is very brief, lasting only the early days of conception, it is important. The single-celled zygote contains all of the genetic information required to form a fetus.

Before fertilization occurs, a number of changes must happen in the sperm in order for it to make its way to the fallopian tube and penetrate the egg. Conditions in the vagina activate ATP enzymes in the sperm. This helps the sperm travel to the fallopian tube.

In addition, lysosomal enzymes are released as the sperm travels. These enzymes are necessary to penetrate the egg’s extracellular matrix. If any of these changes do not occur, the sperm may never make it to the egg or be able to penetrate it.

Once the sperm has entered the ovum, it must digest the egg’s outer membrane so there is a path to the plasma membrane. When a sperm fuses with the egg’s plasma membrane, reactions are triggered that usually prevent another sperm from doing the same.

This is important because it ensures that the correct number of chromosomes are present and prevents a trisomy zygote (a zygote with three sets of chromosomes rather than the usual two).

Timing and hormones also play a role in whether fertilization may occur. A surge in the luteinizing hormone is required in order for ovulation to occur. Progesterone helps prepare a habitable environment for implantation by thickening the lining of the uterus. Inadequate production of these hormones could get in the way of fertilization or implantation.

Timing

The egg and sperm join in the days following ovulation after vaginal sex or medically assisted fertilization. The zygote phase is short, lasting only around four days, after which its cells rapidly divide to become a blastocyst.

The blastocyst develops around the fifth day following fertilization as the zygote travels down the fallopian tube toward the uterus. Once in the uterus, on around day 7, the blastocyst may implant into the endometrium (the lining of the uterine wall). 

Pregnancy in Weeks

It is important to note that pregnancy is counted in weeks, which start on the first day of the person’s last menstrual period before fertilization actually occurs. In pregnancy weeks, a zygote forms during week 3.

Twins

Twins may develop from the same zygote (monozygotic) or different zygotes (dizygotic). Monozygotic twins are called identical, and dizygotic twins are called fraternal.

Monozygotic twins develop when a single, fertilized egg splits and the cells separate into two blastocysts rather than staying together in a single blastocyst.

These twins start with the same chromosomes and often look identical and are identified as the same sex at birth. They may share an amniotic sac and placenta, depending on when they separated.

Dizygotic twins develop when two eggs get fertilized by two sperm. These will go on to produce two embryos. Unlike monozygotic twins, dizygotic twins do not share the same genetic material because they were formed from separate zygotes.

The genetic similarities of dizygotic twins are that of any siblings. These types of twins can be identified as the same or different sexes at birth. They develop in separate sacs and are nourished by separate placentas. Dizygotic twins are the most common type of twinning, making up 70% of twin pregnancies.

Complications

Certain complications can occur during the zygote stage. Chromosomal abnormalities most often occur during fertilization or as a result of a problem with an egg or a sperm. When an abnormality occurs at this stage, it affects every cell of the developing zygote. 

Chromosomal abnormalities can be either numerical or structural. Numerical abnormalities are either missing a chromosome or have too many chromosomes. Some examples include trisomy 21 (also called Down syndrome) and Turner syndrome.

Structural anomalies involve chromosomes whose structure has been altered. Risk factors for chromosomal abnormalities include advanced maternal age and environmental factors.

Trisomy 21

Down syndrome is a condition that results from having an extra chromosome 21. Trisomy is the medical term for having an extra chromosome. The extra chromosome in trisomy 21 affects how the brain and body develop.

Babies born with Down syndrome share some distinct physical features and have certain intellectual disabilities. They are also at increased risk for some health conditions. 

Some defining features of Down syndrome include:

  • Almond-shaped eyes that slant upward
  • Small head, ears, and short neck
  • Protruding tongue
  • Short stature
  • Short limbs
  • Mild to moderately low IQ
  • Slower to learn to speak
  • Low muscle tone
  • At higher risk for hearing loss, sleep apnea, ear infections, vision problems, and heart defects

Turner Syndrome

Turner syndrome affects those assigned female at birth and is the most common sex chromosomal abnormality. It happens when a person is born with one of their X chromosomes missing—either partially or completely.

Some defining features of Turner syndrome include:

  • Short stature
  • Developmental delays
  • Higher risk for heart problems
  • Delayed puberty and lack of sexual development

Turner syndrome can happen for a number of reasons. Sometimes a person with Turner syndrome passes it on to their baby (although most cannot get pregnant naturally).

Forty-five percent of people with Turner syndrome have monosomy X, which results from an egg or sperm that forms without an X chromosome. Thirty percent of Turner syndrome cases are mosaic, in which some cells have two chromosomes while others have only one. This type occurs during cell division in early pregnancy.

Ectopic Pregnancy

An ectopic pregnancy occurs when a fertilized egg continues to develop outside of the uterus, usually in the fallopian tube (which is why it is sometimes referred to as a tubal pregnancy). Ectopic pregnancies are life-threatening, as the fallopian tube can burst as the fertilized egg grows. 

Risk factors for ectopic pregnancy include:

  • Prior ectopic pregnancy
  • Previous surgery on reproductive organs
  • Pelvic inflammatory disease
  • Endometriosis
  • Smoking
  • Advanced maternal age
  • Use of in vitro fertilization (IVF)

Symptoms should be reported to your doctor right away. They include:

  • Vaginal bleeding
  • Pelvic pain or cramping that is mild or severe
  • Weakness or fainting

Ectopic pregnancy is treated with either medication to stop cell growth or surgery to remove the pregnancy from the tube. If the tube has ruptured, emergency surgery will be performed to remove the pregnancy and will often involve removing part or all of the affected tube.

Failed Implantation

Not all zygotes make it to the blastocyst stage. In fact, only around one-third of conceptions make it to live birth. Nearly a third of those losses occur before implantation.

Unless a pregnancy involves fertility assistance, a person will never even know that a zygote has formed when the fertilized egg fails to implant. They will go on to have a normal menstrual cycle. For this reason, failed implantations are not clinically recognized as miscarriages.

Reasons for failed implantation or miscarriage are usually due to chromosomal abnormalities in the zygote. Other reasons include:

  • Infection
  • Exposure to toxins
  • Uterine and cervical abnormalities
  • Underlying health issues

Some risk factors for failed implantation and miscarriage include:

Assisted Reproduction

Assisted reproduction to create a zygote is used for many reasons, including for people who have difficulty getting pregnant, who do not engage in vaginal sex, or who wish to carry a surrogate pregnancy.

Some examples of assisted reproduction include: 

  • Medications to help sperm or egg stimulation to improve the chances of forming a zygote
  • Intrauterine insemination (IUI), where sperm is placed directly in the uterus to meet with an egg and form a zygote
  • In vitro fertilization (IVF), where fertilization occurs outside of the body and the zygote develops into an embryo, which is then placed inside the uterus

Embryo cryopreservation involves freezing embryos after they have been fertilized and grown in a lab for later use. This is often done during in vitro fertilization.

Infertility is treated with medication or surgery 85% to 90% of the time. Only 3% of treatments involve IVF. Success rates vary, depending on the type of treatment and other factors, ranging from as low as 4% to as high as 50%.

A Word From Verywell

The zygote stage is the earliest stage of conception, also known as fertilization. During this stage, you will likely not know whether fertilization has occurred. If fertilization or implantation fails, you’ll simply go on to have your period.

If you are having trouble getting or staying pregnant, remember that you are not alone and there is help. Talk to your doctor about your situation and discuss if fertility options might be best for you.

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Article Sources
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