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A successful pregnancy goes through many distinct stages. A man's sperm should reach, penetrate, and fertilize a woman's egg. The resulting cell must divide and form a blastocyst. The blastocyst much reach the uterus and implant in the mucous membrane. The established blastocyst continues its development into an embryo then a fetus.
At any point during this process issues can occur that interfere with a successful pregnancy.
1. Fertilization
Fertilization is the union of the feminine gamete (egg) and the male gamete (spermatozoa). whether or not it happens naturally within the feminine reproductive system or with the help of reproductive technologies outside of the physical body, the product is a structure referred to as a zygote.
When a woman is ovulating she releases one egg into her Fallopian tubes (or additional in the case of fraternal twins). throughout this time, a woman's cervical mucus will thin, in preparation for sperm to pass through more effectively.
Following spermatozoa ejaculation within the vagina, special secretions facilitate them to swim through the cervix towards the uterine tube where fertilization takes place within 24-72h1.
The fertilized egg or zygote then begins to maneuver toward the uterus, because the cells divide into the next stage, a blastocyst.
Problems with Fertilization
There are a wide range of issues that may prevent fertilization, however all of them have the same outcome. one thing is stopping sperm and egg from reaching one another.
Absence of Sperm/Egg: If a woman has an annovulatory cycle where she does not release an egg, or if no sperm reaches the egg because of azoospermia or low sperm count, then fertilization will not occur.
Sperm's Ability to reach the Egg: Poor sperm motility or movement, or poor chemotaxis² (sperms ability to navigate toward the egg) will cause issues. Similarly, if a woman's cervical mucous secretion doesn't thin enough during ovulation, it's going to not let sperm reach the egg.
Sperm Penetration: Poor morphology, abnormal sperm form makes it more durable to penetrate an egg. sperm also depends on a chemical change, referred to as an acrosome reaction, that helps it produce a hole within the egg to pass through. Poor or incomplete acrosome reaction may also inhibit fertilization.
2. blastocyst Development
Soon after fertilization, the embryo is formed from a small cluster of cells that are perpetually dividing inside a complex structure referred to as the blastocyst. it's shaped by two groups of cells, inner and outer cells, and fluids. The blastocyst stays within a protective cover during maturation referred to as zona pellucida, that can be delineated as an egg shell. The outer cells are situated right below this cover, which is able to produce the future placenta and surrounding tissues to support fetal development within the womb. The inner cells of the blastocyst can become the various tissues and organs of the human body, like bones, muscles, skin, liver, and heart4,5.
The cells within the blastocyst grow quick, they are going through several changes and convert into additional specialised cells, creating the structure very tight. In humans, these changes happen throughout the first few days of development, before the implantation within the uterus. At this stage, the zona pellucida (similar to an egg shell) breaks and releases the blastocyst. It moves through the Fallopian tubes towards the womb and implants around day ten4,5.
issues With blastocyst Development
Blastocyst arrest is that the term for when the cells fail to divide stopping the developmental progress of the embryo. whereas the precise causes of blastocyst arrest aren't totally understood, they're generally associated with genetic abnormalities within the sperm or egg.
3. blastocyst Implantation
When the blastocyst reaches the uterus it implants within the mucous membrane, the mucus membrane that lines the uterus. The external cells of the blastocyst and therefore the female internal reproductive organ inner lining, together, can produce the future placenta. The placenta is a structure that transfers nutrients to the baby and removes his/her wastes6.
Problems with Implantation
When a fertilized egg is able to implant within the womb however fails to develop it will be described as an empty gestational sac or """"blighted ovum"""". It typically occurs due to the presence of abnormalities in the chromosomes of the sperm, the egg cell or the fertilized egg or cellular division. This event could happen within the early days of pregnancy7.
Sometimes, a fertilized egg won't implant within the womb. there's scientific proof that the endometrium is to blame for the selection of the embryo before facilitating the implantation. This mechanism eventually leads to implantation of healthy embryos or rejection of abnormal embryos. Some human embryos have alterations in their genes referred to as mutations that may delay or impede normal development. These impairments make them inappropriate for natural implantation and can increase the probability for miscarriage8.
4. Embryo Development
As the blastocyst reaches the final steps within the implantation process into the inner lining of the womb, it evolves into a structure referred to as an embryo. this is often the time when internal organs and external structures develop. The mouth, lower jaw, throat square measure emerging, whereas the blood circulation system starts its evolution and a heart tube is formed. The ears arise and arms, legs, fingers, toes, and eyes are being formed. The brain and therefore the spinal cord are already shaped, while the digestive tract and sensory organs start their development. the first bones are replacing the cartilage. after ten-twelve weeks of pregnancy, the embryo moves into the final stage of development, a fetus.
Problems with Embryo Development
Severe issues with embryonic development can occur as early because the first three weeks. With so many organ systems and external structures starting specialization and development in the embryo, the developing organism is especially sensitive to damage from genetic abnormalities and environmental exposures. These genetic abnormalities will range from spontaneous genetic mutations, to improper body arrangement that may potentially have an effect on the development of vital structures just like the heart or brain.
Alongside genetic abnormalities, any variety of environmental factors (malnutrition, infections, disease, toxic exposures) can have a bearing on genetic expression potentially proving to be fatal to the developing embryo.
5. fetal Development
By the twelfth week of fertilization the embryo moves into the final stage of development referred to as the fetal stage. By now, the fetus has formed all of the organs and structures necessary for a baby, however those organs still need to grow and develop.
At three months of pregnancy, the higher and lower extremities of the fetus are fully developed. Ears and teeth are shaped and the reproductive organs have evolved. At the end of this month, the fetus has completed the growth of most of the circulatory and urinary systems and its length is about five inches.
At six months, the fetus will respond to sounds and is around twelve inches long. The fetus continues to develop and grow changing position and responding to sounds, and stimuli. The length of the tiny body can reach fourteen inches. The fetus becomes a baby at month eight. The brain is evolving quickly therefore the baby can see and hear, although the respiratory system needs maturation. The baby will weigh more or less five pounds. near the end of the pregnancy at month nine, the baby responds to stimuli, will move the whole body however space around the new human being is turning into too tight. If the baby had not changed his/her position within the womb, this is the proper time to drop down into the pelvis towards the birth canal to face the world nine.
Problems with fetal Development
Because all major structures are already formed within the fetus, the fetus isn't as sensitive as the embryo to damage from environmental exposures. this is often why after the primary trimester, a miscarriage is far less possible. However, toxic environmental exposures can contribute to physiological abnormalities or minor congenital malformations.
Keywords: Embryo Development
