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. Show 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. 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 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. 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.
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. 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. 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. 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.
A blastocyst is a cluster of dividing cells made by a fertilized egg. It’s the early stage of an embryo. A blastocyst is one step among many that lead to a pregnancy. A blastocyst forms about five to six days after a sperm fertilizes an egg. Layers of cells in the blastocyst divide and separate. They eventually become the structures that protect and nourish the developing fetus (unborn baby). The blastocyst stage of a fertilized egg is especially important for in vitro fertilization (IVF). IVF is the process of creating an embryo outside of the birthing parent’s body to assist with pregnancy. What are the steps in fertilization and embryonic development?To understand the role of a blastocyst, it helps to know how ovulation, conception and pregnancy occur. About 14 days after a person begins their menstrual period, their ovaries release one egg (ovum). The egg goes into one of their fallopian tubes, where it waits for fertilization by a sperm. If fertilization doesn’t occur, the egg moves into their womb (uterus) where it exits their body during their next menstrual period. But if fertilization does occur, their fertilized egg becomes a zygote, a single cell with genes from both parents. The zygote travels through their fallopian tube to their uterus about three to five days after fertilization. The cell of the zygote continuously divides, eventually forming a hollow ball of cells called the blastocyst. The blastocyst stays in their uterus for several days before it implants in the inner lining of their uterine wall (endometrium). It continues to make new cells, which separate into layers. About 10 to 12 days after fertilization, the blastocyst develops into an embryo. It remains an embryo until about nine weeks after implantation, when it then becomes a fetus.
The blastocyst stage is an extremely important part of embryonic and fetal development. If the blastocyst doesn’t implant in the person's endometrium, pregnancy will not occur. For implantation to occur, hormones trigger a process called hatching. The blastocyst sheds its clear outer membrane. Hatching occurs one to three days after a blastocyst enters their uterus. Next, cells on the outer layer of the blastocyst attach to the outer layer of their endometrium. They release a sticky protein called L-selectin, which binds with substances in their endometrium. These outer cells eventually become the fetus. Cells in the inner layer of the blastocyst implant more deeply in their endometrium. These cells become the placenta, which carries oxygen and nutrients to the developing fetus. Some of the placental cells also form the amniotic sac. The amniotic sac contains a clear, protective fluid that surrounds the fetus as it develops. Where does blastocyst implantation occur?Implantation usually happens toward the top of the person's uterus. In rare cases, a blastocyst never makes it to their uterus. Instead, it implants outside their uterus (usually in a fallopian tube), creating an ectopic pregnancy. What are the signs of blastocyst implantation?About one-third of people experience some light bleeding or spotting during implantation. Spotting may be the first sign of pregnancy. The blood tends to be light pink or dark brown and only lasts for a few hours to a few days, so it won’t be similar to your period. Other signs may include: Why is the blastocyst so important for IVF?During IVF, healthcare providers evaluate and grade blastocysts in a laboratory to determine which embryos are most likely to lead to a pregnancy. Five or six days after fertilization, blastocysts are ideal to transfer to your uterus during IVF. Healthcare providers may grade the blastocysts based on their maturity, shape, cell number and density.
A blastocyst is a hollow ball of cells. The cells form two layers. The inner layer is about three to four cells thick and the outer layer is about one cell thick. The cells in a blastocyst divide rapidly. A mature blastocyst may contain as many as 200 to 300 cells.
Blastocysts contain chromosomes. If there are chromosomal abnormalities, such as an extra chromosome or a chromosome that’s duplicated or deleted, the blastocyst usually won’t implant in your endometrium. Failed implantation may lead to an early miscarriage at about five weeks, often called a “chemical pregnancy.” If you haven’t missed your period or taken a pregnancy test, you may not even know you had a chemical pregnancy. If the blastocyst does implant and leads to a successful pregnancy, there is always a risk that the baby may have a chromosomal disorder such as:
How often do blastocysts fail to implant?Failure for a blastocyst to implant is one of the most common reasons for unsuccessful IVF treatments. It’s also the cause of about 75% of early miscarriages. About half of all early miscarriages are the result of chromosomal abnormalities in embryos. What are the symptoms of a failed implantation?You may not know if a blastocyst fails to implant. A lot of people don’t have any symptoms. Some experience light spotting or bleeding. Since these symptoms are similar to those of successful implantation, it’s important to talk to your healthcare provider so they can test for pregnancy. Can chromosomal abnormalities be diagnosed at the blastocyst stage?During IVF, healthcare providers can do preimplantation genetic testing (PGT). They take cells from an embryo at the blastocyst stage to check for chromosomal abnormalities. They perform tests in a lab to identify which embryos are healthy and have the greatest chance of implanting once they’re transferred to your uterus. If you get pregnant naturally, prenatal testing for chromosomal abnormalities happens after the blastocyst stage. The earliest tests usually take place nine to 10 weeks into a pregnancy.
Many risk factors for chromosomal abnormalities aren’t preventable, such as:
If you’re at high risk due to any of these factors, talk with your healthcare provider and a genetic counselor if you plan to get pregnant. You can lower your risk of causing chromosome damage in an embryo by practicing a healthy lifestyle before and during pregnancy:
A note from Cleveland Clinic Blastocysts are part of early embryonic development and play a crucial role in a successful pregnancy. Blastocysts that don’t develop normally or don’t implant in the womb won’t lead to a pregnancy. If you’re having IVF, your healthcare provider can test blastocysts to see if they’re healthy. You might experience light bleeding or mild cramping very early in a pregnancy as the blastocysts form and attach to your womb. But call your healthcare provider right away if you notice more serious symptoms.
Last reviewed by a Cleveland Clinic medical professional on 04/29/2022. References
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The zygote divides and implants into the wall of the uterus during which stage of gestation?
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