embryology

 

Pregnancy 

Pregnancy is the fertilization and development of one or more offspring, known as an embryo or fetus, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets. Childbirth usually occurs about 38 weeks after conception; in women who have a menstrual cycle length of four weeks, this is approximately 40 weeks from the start of the last normal menstrual period (LNMP). Human pregnancy is the most studied of all mammalian pregnancies. Conception can be achieved through sexual intercourse or assisted reproductive technology.

An embryo is the developing offspring during the first 8 weeks following conception, and subsequently the term fetus is used henceforth until birth.  40% of pregnancies in the United States and United Kingdom are unplanned, and between a quarter and half of those unplanned pregnancies were unwanted pregnancies.

In many societies’ medical or legal definitions, human pregnancy is somewhat arbitrarily divided into three trimester periods, as a means to simplify reference to the different stages of prenatal development. The first trimester carries the highest risk of miscarriage (natural death of embryo or fetus). During the second trimester, the development of the fetus can be more easily monitored and diagnosed. The beginning of the third trimester often approximates the point of viability, or the ability of the fetus to survive, with or without medical help, outside of the uterus.

First Trimester

Weeks 0 to 12

At the beginning of your first trimester, the first month of pregnancy, a water-tight sac, known as the amniotic sac, begins to develop around the fertilized egg in your uterus. The placenta then begins to develop to help provide nutrients to your growing embryo. The placenta is also instrumental in removing wasted from your baby.

Even in the first month your baby will begin to develop a tiny little face, with dark black circles for eyes. Cells begin to take shape developing body parts such as the mouth, throat and lower jaw. Circulatory systems become active, powering massive cell growth. At the end of the first month your baby will still be tiny, only about the size of a piece of rice or about a quarter inch.

During the second month your baby’s facial features will continue to develop. Small folds on the sides of the head will soon become tiny little ears. Small bulbs will begin to develop on the side of the embryo which will become future arms and legs. Fingers and toes will also begin to develop at the ends of those arms and legs.

The neural tube will begin to develop which eventually becomes your baby’s brain and spinal cord. The digestive system and other sense organs also begin development. Cartilage starts to become bone and your little embryo may even start moving, although it’s impossible to feel at this point. By the end of the second month your embryo will have grown to about 1 inch long, with 1/3 of that being the head alone.

In the last month of the first trimester, your baby will already be fully formed complete with tiny arms, legs, hands, feet, fingers and toes. Your baby will even be able to open and close its hands and mouth by this point. External ears will begin to spring out and things like fingernails, toenails and teeth are all well on their way. Although sexual organs will have developed by the end of the first trimester, it’ll be almost impossible to detect on an ultrasound.

Second Trimester

Weeks 13 to 28

By the first week of your second trimester your baby’s features have become well defined. Things like eyelids, eyelashes and hair begin to form. Bones become stronger during the second trimester including teeth. Your baby will be able to do things like suck their thumb, stretch or even yawn.

The nervous system becomes fully active during the second trimester. The reproductive organs and genitals have become full formed, allowing you to determine the sex of your baby for the first time using an ultrasound. Your baby’s heart will be pumping strong and can even be listened to using a device known as a fetal Doppler. By the end of your fourth month of pregnancy your baby will have grown to be about 6 inches long and will weigh about a quarter pound.

By the fifth month of pregnancy, in the middle of your second trimester, your baby will develop soft peach fuzz hair that usually covers the back and shoulders and can even cover parts of the face. This can make your little one look a little like Sasquatch when they first come out, but don’t worry. This hair is typically shed in the first week or two of your baby’s life. Your baby’s skin will also become covered in a white coating known as vernix caseosa that’s thought to protect your baby from the amniotic fluid in the womb. Fortunately, this cheesy substance is shed before birth. By the end of the fifth month your baby will be about 10 inches long and could weigh up to a pound.

By the end of the second trimester, in the sixth month, your baby will be about 12 inches long and could weigh up to 2 pounds. Their skin will be kind of wrinkled and reddish with visible veins appearing through a translucent layer of skin. Fingers and toes are visible and you can even start to see little fingerprints. The best part about the end of the second trimester is that you will finally be able to feel your baby move. Your may even feel a slight jerking sensation that’s caused by baby hiccups.

Third Trimester

Weeks 29 to 40

Your third trimester begins during your seventh month of pregnancy. During this month fat begins to form in your baby’s body. Your baby may be up to 14 inches or longer at this point and could weight anywhere from 2 to 5 pounds. At this stage your baby’s hearing is fully developed, so it’s important to be sensitive to this fact. Your baby will begin to respond to things such as sound, pain and even light at this stage of pregnancy.

The eighth month of pregnancy is very similar to the seventh. Your baby will continue to build fat reserves which will allow them to function free of mommy’s womb. Your baby will be up to 18 inches or longer at this point and will weigh up to five pounds. Multiples will weigh slightly less at this point but will be at the same point of development as other babies. While most internal systems have been fully developed by this point, the lungs may still not be fully mature.

The end of the third trimester marks your final month of pregnancy. During this month your baby will continue to grow and develop, although there should be significantly less weight gain this month. At this stage your baby will be able to blink, turn their head and grasp using a tiny fist. This month you may notice an increased sensitivity in your baby to outside stimulation like light, sound and touch.

At the end of the third trimester your baby will begin to position herself to prepare to delivery. Your baby will drop down into your pelvis with her head facing towards the birth canal. If her head isn’t facing the birth canal, a Caesarian section (c section) may be necessary. By the end of the third trimester your baby will be 15 to 20 inches long and could weight from 6 to 8 pounds or more.

Ectopic pregnancy

An ectopic pregnancy is a complication of pregnancy in which the embryo implants outside the uterine cavity. With rare exceptions, ectopic pregnancies are not viable. Most ectopic pregnancies occur in the Fallopian tube (so-called tubal pregnancies), but implantation can also occur in the cervix, ovaries, and abdomen. It should be considered in any woman with abdominal pain or vaginal bleeding who has a positive pregnancy test. An ultrasound showing a gestational sac with fetal heart in a location other than the uterine cavity is clear evidence of an ectopic pregnancy. Tubal ectopic pregnancy is the most common cause of maternal death in the first trimester of pregnancy.

About 1% of pregnancies are in an ectopic location with implantation not occurring inside of the womb, and of these 98% occur in the Fallopian tubes. In a typical ectopic pregnancy, the embryo adheres to the lining of the fallopian tube and burrows into the tubal lining. Most commonly this invades blood vessels which causes bleeding resulting in the expulsion of the implantation from the tube. Termed "tubal abortions", about half of ectopic pregnancies will resolve without treatment. The use of methotrexate treatment for ectopic pregnancy has reduced the need for surgery, but surgical intervention is still required in cases where the Fallopian tube has ruptured or is in danger of doing so. The surgical intervention may be laparoscopic or through a larger incision, known as a laparotomy.

A woman who has had a previous ectopic pregnancy is more likely to have another. The majority of women with ectopic pregnancies have had pelvic inflammatory disease or salpingitis, an inflammation of the fallopian tube. A history of gonorrhea or chlamydia can also cause tubal problems that increase the risk. Endometriosis, a condition that causes the tissue that normally lines the uterus to develop outside the uterus may slightly increase the incidence of an ectopic. The risk is increased in women who have unusually shaped fallopian tubes or tubes which has been damaged, possibly during surgery. Taking medication to stimulate ovulation increases the risk of ectopic pregnancy. Although pregnancy is rare when using birth control pills or an intrauterine device (IUD), if it does occur, it's more likely to be ectopic. Although pregnancy is rare after tubal ligation, if it does occur, it's more likely to be ectopic. A recent meta-analysis of clinical outcomes has shown that cigarette smoking significantly increases the risk of tubal ectopic pregnancy.

Congenital disorder

 A congenital disorder, or congenital disease, is a condition existing at birth and often before birth, or that develops during the first month of life (neonatal disease), regardless of causation. Of these diseases, those characterized by structural deformities are termed "congenital anomalies"; that is a different concept (MeSH) which involves defects in or damage to a developing fetus.

A congenital disorder may be the result of genetic abnormalities, the intrauterine (uterus) environment, errors of morphogenesis, infection, or a chromosomal abnormality. The outcome of the disorder will depend on complex interactions between the pre-natal deficit and the post-natal environment. Animal studies indicate that the mother's (and possibly the father's) diet, vitamin intake, and glucose levels prior to ovulation and conception have long-term effects on fetal growth and adolescent and adult disease. Congenital disorders vary widely in causation and abnormalities. Any substance that causes birth defects is known as a teratogen.

The older term congenital  disorder does not necessarily refer to a genetic disorder despite the similarity of the words. Some disorders can be detected before birth through prenatal diagnosis (screening).

Classification

Much of the language used for describing congenital conditions predates genomic mapping, and structural conditions are often considered separately from other congenital conditions. It is now known that many metabolic conditions may have subtle structural expression, and structural conditions often have genetic links. Still, congenital conditions are often classified in a structural basis, organized when possible by primary organ system affected.

Several terms are used to describe congenital abnormalities. (Some of these are also used to describe noncongenital conditions, and more than one term may apply in an individual condition.)

    A congenital physical anomaly is an abnormality of the structure of a body part. An anomaly may or may not be perceived as a problem condition. Many, if not most, people have one or more minor physical anomalies if examined carefully. Examples of minor anomalies can include curvature of the 5th finger (clinodactyly), a third nipple, tiny indentations of the skin near the ears (preauricular pits), shortness of the 4th metacarpal or metatarsal bones, or dimples over the lower spine (sacral dimples). Some minor anomalies may be clues to more significant internal abnormalities.
    Birth defect is a widely used term for a congenital malformation, i.e. a congenital, physical anomaly which is recognizable at birth, and which is significant enough to be considered a problem. According to the CDC, most birth defects are believed to be caused by a complex mix of factors including genetics, environment, and behaviors, though many birth defects have no known cause. An example of a birth defect is cleft palate.
    A congenital malformation is a congenital physical anomaly that is deleterious, i.e. a structural defect perceived as a problem. A typical combination of malformations affecting more than one body part is referred to as a malformation syndrome.
    Some conditions are due to abnormal tissue development:
        A malformation is associated with a disorder of tissue development. Malformations often occur in the first trimester.
        A dysplasia is a disorder at the organ level that is due to problems with tissue development.
    It is also possible for conditions to arise after tissue is formed:
        A deformation is a condition arising from mechanical stress to normal tissue. Deformations often occur in the second or third semester, and can be due to oligohydramnios.
    A disruption involves breakdown of normal tissues.
    When multiple effects occur in a specified order, it is known as a sequence. When the order is not known, it is a syndrome.

Other

    Genetic disorders or diseases are all congenital, though they may not be expressed or recognized until later in life. Genetic diseases may be divided into single-gene defects, multiple-gene disorders, or chromosomal defects. Single-gene defects may arise from abnormalities of both copies of an autosomal gene (a recessive disorder) or of only one of the two copies (a dominant disorder). Some conditions result from deletions or abnormalities of a few genes located contiguously on a chromosome. Chromosomal disorders involve the loss or duplication of larger portions of a chromosome (or an entire chromosome) containing hundreds of genes. Large chromosomal abnormalities always produce effects on many different body parts and organ systems.
    A congenital metabolic disease is also referred to as an inborn error of metabolism. Most of these are single gene defects, usually heritable. Many affect the structure of body parts but some simply affect the function.

Aging and Death 

 Diseases Associated With Aging, Death

Aging is the natural effect of time and the environment on living organisms, and death is its end result. Gerontology is the study of all aspects of aging. No single theory on how and why people age is able to account for all facets of aging. Although great strides have been made to postpone death as the result of certain illnesses, less headway has been made in delaying aging.

Life span is species-specific. Members of the same species have similar life expectancies. In most species, death occurs not long after the reproductive phase of life ends. This is obviously not the case for humans. However, there are some changes that occur in women with the onset of menopause when estrogen levels drop. Post-menopausal women produce less facial skin oil (which serves to delay wrinkling) and are at greater risk of developing osteoporosis (brittle bones). Men continue to produce comparable levels of facial oils and are thus less prone to early wrinkling. Osteoporosis occurs as calcium leaves bones and is used elsewhere; hence, sufficient calcium intake in older women is important because bones which are brittle break more easily.

Aging and Death

The relationship between aging and death is complex. The results from many studies indicate that aging decreases the efficiency of the body to operate, defeat infections, and to repair damage. Comparison of people aged 30–75 has demonstrated that the efficiency of lung function decreases by 50% that bones become more brittle, and that the immune system that safeguards the body from infections generally becomes less efficient as we age.

Why this deterioration in the functioning of the body with age occurs is still not clear. Several theories have been proposed to explain this decline. One theory proposes that after the active years of reproduction have passed, chemical changes in the body cause the gradual malfunctioning of organs and other body components. The accumulation of damage to components that are necessary for the formation of new cells of the body leads to death. For example, it has been discovered that the formation of the genetic material deoxyribonucleic acid (DNA) is more subject to mistakes as time goes on. Other theories relating aging with death include the negative effect of stresses to the body, and a theory that proposes that the buildup of non-functional material in the body over time lessens the ability of the body to function correctly.

The strongest arguments on the aging process favor involvement of one of, or a combination of the following: hormonal control, limited cell division, gene theory, gene mutation theory, protein cross-linkage theory, and free radical action. In support of hormonal control, there is the observation that the thymus gland (under the sternum) begins to shrink at adolescence, and aging is more rapid in people without a thymus. Another hormonal approach focuses on the hypothalamus (at the base of the brain), which controls the production of growth hormones in the pituitary gland. It is thought that the hypothalamus either slows down normal hormonal function or that it becomes more error-prone with time, eventually leading to physiological aging.