A zygote, also known as a fertilized ovum or fertilized egg, is the union of a sperm cell and an egg cell. The zygote begins as a single cell but divides rapidly in the days following fertilization. The zygote’s single cell contains all of the 46 necessary chromosomes, getting 23 from the sperm and 23 from the egg.
The zygote phase is brief, lasting only about four days. Around the fifth day, the mass of cells becomes known as a blastocyst. The embryo develops from the blastocyst.
How Zygotes Form
In order for reproduction to take place, a single sperm cell must penetrate the outer surface of an egg in a process known as fertilization. During a healthy reproductive cycle, a single egg cell is released from the follicle into the fallopian tube at ovulation.
If sperm are present, thousands will attempt to penetrate this single egg cell. Once a single sperm has broken through the outer surface, a zygote is formed. Chemical changes in the surface of the egg prevent other sperm from entering.
Medically Assisted Fertilization
Medically assisted fertilization is also possible and becoming increasingly common. Intrauterine insemination (IUI) and in vitro fertilization (IVF) are two frequently used assisted reproductive techniques.
During IUI, semen is inserted into the uterus using a catheter with fertilization taking place inside the body. With IVF, eggs are removed from the ovaries and fertilized in a lab. The blastocyst is then implanted in the uterus.
When a Zygote Becomes an Embryo
Zygotes divide through a process known as mitosis, in which each cell doubles (one cell becomes two, two becomes four, and so on). This two-week stage is known as the germinal period of development and covers the time of fertilization (also called conception) to the implantation of the blastocyst in the uterus.
The sperm cell contains paternal genetic information while the egg cell contains maternal genetic information. Because each cell contains half of the genetic material, each cell is known as a haploid cell. When these two haploid cells join, they form a single diploid cell that contains all necessary chromosomes.
The zygote then travels down the fallopian tube to the uterus. As it travels, its cells rapidly divide and it becomes a blastocyst. Once in the uterus, the blastocyst must implant in the lining in order to obtain the nourishment it needs to grow and survive.
The embryonic period of development lasts from two weeks after conception through the eighth week, during which time the organism is known as an embryo. At the ninth week post-conception, the fetal period begins. From this point until birth, the organism is known as a fetus.
Zygotes in Twinning
Identical twins are monozygotic. With monozygotic twins, one egg is fertilized and one zygote is formed, but at the blastocyst phase, it splits to form two embryos. Monozygotic twins share the same genetic material.
A Note About Conception
Conception occurs when an egg is fertilized, but pregnancy does not actually begin until a blastocyst implants into the uterus. It’s not usually possible to know whether fertilization has occurred at this early stage, considered week 3 of pregnancy. Symptoms and pregnancy hormone levels are usually not notable until week 4 or 5.
Fraternal twins, on the other hand, are dizygotic, which means that two eggs are fertilized resulting in two zygotes. Those two zygotes go on to develop into two embryos. Unlike monozygotic twins, dizygotic twins do not share identical genes.
Potential Challenges
Not all zygotes make it to the next stage of prenatal development. Researchers estimate that 30% to 70% of all naturally occurring conceptions fail either before or at the time of implantation. Researchers suspect these losses are connected to abnormalities. In cases of recurrent miscarriage, a parental chromosomal anomaly is often to blame.
In the case of these very early miscarriages, also known as chemical pregnancies, a person may not be aware that fertilization had occurred because they may experience bleeding similar to and around the time of their expected menstrual period. With the advent of early result home pregnancy tests, however, more people are able to detect chemical pregnancies as early as four or five days before their expected menstrual cycle.
IUI and IVF can fail as well. Success rates for IUI range from 7% to 20%. Studies have linked poor semen parameters with IUI failure. Poor quality eggs and hormone deficiencies are other known reasons for IUI failure.
IVF success rates vary by age, with the greatest chance of success in parents who are younger than 35. At the younger end of the spectrum, the chance of success with IVF is around 54% but those numbers diminish over time, with success rates at only 4% by age 43. Things that can influence IVF success or failure include a parent’s age, prior pregnancies and losses, the viability of the eggs, and the underlying cause of infertility.
