Pregnancy What Is a Zygote? The Single-Cell Stage After the Egg Is Fertilized By Kathi Valeii Kathi Valeii Kathi Valeii is a freelance writer covering the intersections of health, parenting, and social justice. Learn about our editorial process Updated on October 16, 2021 Medically reviewed by Monique Rainford, MD Medically reviewed by Monique Rainford, MD Monique Rainford, MD, is board-certified in obstetrics-gynecology, and currently serves as an Assistant Clinical Professor at Yale Medicine. She is the former chief of obstetrics-gynecology at Yale Health. Learn about our Medical Expert Board Print Table of Contents View All Table of Contents Formation Timing Twins Complications Assisted Reproduction Frequently Asked Questions 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. 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. The Anatomy of the Fallopian Tubes 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. The Endometrium and Its Role in Reproductive Health 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 upwardSmall head, ears, and short neckProtruding tongueShort statureShort limbsMild to moderately low IQSlower to learn to speakLow muscle toneAt 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 statureDevelopmental delaysHigher risk for heart problemsDelayed 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 pregnancyPrevious surgery on reproductive organsPelvic inflammatory diseaseEndometriosisSmokingAdvanced maternal ageUse of in vitro fertilization (IVF) Symptoms should be reported to your healthcare provider right away. They include: Vaginal bleedingPelvic pain or cramping that is mild or severeWeakness 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. What Is an Ectopic Pregnancy? 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: InfectionExposure to toxinsUterine and cervical abnormalitiesUnderlying health issues Some risk factors for failed implantation and miscarriage include: Advanced maternal age Hormonal irregularities Smoking, alcohol use, and drug use Health conditions like kidney disease, heart disease, and thyroid disease 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 zygoteIntrauterine insemination (IUI), where sperm is placed directly in the uterus to meet with an egg and form a zygoteIn 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%. How Intrauterine Insemination Works 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 healthcare provider about your situation and discuss if fertility options might be best for you. Frequently Asked Questions How many chromosomes does a human zygote have? The human zygote, or a fertilized egg, has 46 chromosomes. This means that 23 chromosomes are from the egg, and 23 chromosomes are from the sperm. A zygote consists of how many cells? For the first 12 hours after conception, a zygote (fertilized egg) is one cell. Around 30 hours later, it divides into two cells. Around 15 or so hours after that, those two cells divide to become four cells. When three days have passed, the zygote will consist of 16 cells. At that point, it is called a morula. What is the difference between monozygotic and dizygotic twins? Monozygotic twins, or identical twins, are formed by a single zygote that splits itself into two blastocysts. These twins share the same genetic material.Dizygotic twins, or fraternal twins, are formed by two different zygotes fertilized by two sperm. These separate zygotes go on to form embryos. These twins do not share the same genetic material. Learn More: How Monozygotic Twins Develop 12 Sources Verywell Health 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. Oliver R, Basit H. Embryology, fertilization. StatPearls. Gill P, Lende MN, Van Hook MD JW. Twin births. StatPearls. National Human Genome Research Institute. Chromosome abnormalities fact sheet. Centers for Disease Control and Prevention. Facts about Down syndrome. Cleveland Clinic. Turner syndrome: causes, symptoms, diagnosis & treatment. Eunice Kennedy Shriver National Institute of Child Health and Human Development. Turner syndrome: Other FAQs. American College of Obstetrics and Gynecologists. Ectopic pregnancy. Larsen E, Christiansen O, Kolte A, Macklon N. New insights into mechanisms behind miscarriage. BMC Med. 2013;11(1). doi:10.1186/1741-7015-11-154 Cleveland Clinic. Miscarriage: risks, symptoms, causes & treatments. American Society for Reproductive Medicine. Quick facts about infertility. Society for Assisted Reproductive Technology. National summary report. MedlinePlus. Cell division. By Kathi Valeii As a freelance writer, Kathi has experience writing both reported features and essays for national publications on the topics of healthcare, advocacy, and education. The bulk of her work centers on parenting, education, health, and social justice. See Our Editorial Process Meet Our Medical Expert Board Share Feedback Was this page helpful? 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