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. 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:. Symptoms should be reported to your healthcare provider right away. They include:. 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.
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:. Some risk factors for failed implantation and miscarriage include:. 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:. 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. 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 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.
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. 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. 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. Sign up for our Health Tip of the Day newsletter, and receive daily tips that will help you live your healthiest life.
Oliver R, Basit H. Embryology, fertilization. Updated July 10, Twin births. Updated August 10, National Human Genome Research Institute. Chromosome abnormalities fact sheet. Updated August 15, Centers for Disease Control and Prevention. Facts about Down syndrome. Updated December 28, Skip to main content. Human Development and Pregnancy. Search for:. First Week of Development. Fertilization Fertilization occurs when a sperm and an egg have fused together to form a zygote, which begins to divide as it moves towards the uterus.
Learning Objectives Describe human fertilization. Key Takeaways Key Points Fertilization is commonly known as conception. Once the fertilized gamete ovum implants itself in the uterine lining, pregnancy begins. The fusion of male and female gametes sperm and ovum, respectively usually occurs following the act of sexual intercourse. However, artificial insemination and in vitro fertilization have made achieving pregnancy possible without engaging in sexual intercourse.
Prior to fertilization, sperm undergo a process of capacitation in response to conditions in the female reproductive tract, which include increases in motility and destabilization of the cell membrane that allows the head of the sperm to penetrate the egg.
Key Terms fertilization : The act of fecundating or impregnating animal or vegetable gametes. Nondisjunction : Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. Cleavage of the Zygote The process of cleavage is the step of embryogenesis where the zygote divides to produce a cluster of cells known as the morula.
Learning Objectives Describe cleavage of the zygote. The different cells derived from cleavage, up to the blastula stage, are called blastomeres. For species such as humans, there is little yolk in eggs, and the divisions are relatively symmetrical, or holoblastic.
For other species, such as birds and reptiles, the presence of yolk dictates uneven meroblastic divisions that produce cells of uneven size and distribution. The cells of the morula are at first closely aggregated, but quickly become arranged into an outer or peripheral layer, the trophoblast, which does not contribute to the formation of the embryo proper, and an inner cell mass from which the embryo develops.
Key Terms cleavage : In embryology, this is the division of cells in the early embryo. Blastocyst Formation The blastocyst forms early in embryonic development and has two layers that form the embryo and placenta.
Learning Objectives Outline the process of blastocyst formation. Key Takeaways Key Points The human blastocyst possesses an inner cell mass ICM , or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast, which later forms the placenta.
Before gastrulation, the cells of the trophoblast become differentiated into two strata: the ectoderm of the chorion plays a role in the development of the placenta, and the endoderm differentiates and quickly assumes the form of a small sac, called the yolk sac. The floor of the amniotic cavity is formed by the embryonic disk, which is composed of a layer of prismatic cells, and the embryonic ectoderm, which is derived from the inner cell mass and lies in opposition to the endoderm.
Key Terms embryonic disk : The floor of the amniotic cavity is formed by the embryonic disk or disc that is composed of a layer of prismatic cells called the embryonic ectoderm. It is the part of the inner cell mass from which the embryo is developed. Implantation Implantation is the very early stage of pregnancy at which the embryo adheres to the wall of the uterus and begins to form the placenta. Learning Objectives Outline the process of implantation in pregnancy.
Key Takeaways Key Points At this stage of prenatal development the embryo is a blastocyst. In humans, implantation of a fertilized ovum occurs between 6 to 12 days after ovulation. In preparation for implantation, the blastocyst sheds its outside layer, the zona pellucida, and is replaced by a layer of underlying cells called the trophoblast. The syncytiotrophoblast then implants the blastocyst into the endometrium by forming finger-like projections into the uterine wall called chorionic villi.
Key Terms endometrium : The mucous membrane that lines the uterus in mammals and in which fertilized eggs are implanted. Licenses and Attributions.
CC licensed content, Shared previously. When they join together to form a zygote — aha! To encourage this, the mucus in the cervix becomes more liquid and elastic, allowing sperm to reach the uterus quickly.
Once those sperm show up, a few hundred will try to penetrate into the egg. Only one lucky one will get in. Once it does, chemical changes in the egg itself occur so that no other sperm can break through.
A zygote is formed. Sometimes, your ovaries release more than one egg. If both become fertilized, then congrats! Women with moms or sisters who have had these types of twins are about twice as likely to have them themselves.
In intrauterine insemination IUI , a doctor uses a thin catheter to inject sperm directly into your uterus to increase the chances that sperm will meet the egg. During in vitro fertilization IVF , your eggs are fertilized by sperm in a laboratory at a fertility clinic. Then at least one embryo is transferred into your uterus with the hope that it will spark at least one and possibly more! It takes about five to six days for a zygote to transform into a blastocyst a microscopic ball of cells and then into an embryo.
Within hours after sperm meets egg, the zygote divides and then continues to divide and divide. The blastocyst now begins its big journey from your fallopian tube to your uterus. Very rarely — about three or four out of 1, births — the zygote splits in half, leading to two embryos. Since the original egg was fertilized by one sperm, the genetic material in both embryos is identical, resulting in you guessed it identical twins.
Thankfully, the ones who do are usually sturdy enough to make it through. Once the blastocyst reaches your uterus and attaches to the lining, this little ball of cells does another great divide.
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