When is prenatal period

The head develops in the fourth week and the precursor to the heart begins to pulse. In the early stages of the embryonic period, gills and a tail are apparent.

But by the end of this stage, they disappear and the organism takes on a more human appearance. About 20 percent of organisms fail during the embryonic period, usually due to gross chromosomal abnormalities. As in the case of the germinal period, often the mother does not yet know that she is pregnant.

It is during this stage that the major structures of the body are taking form making the embryonic period the time when the organism is most vulnerable to the greatest amount of damage if exposed to harmful substances. We will look at this in the section on teratology below. Potential mothers are not often aware of the risks they introduce to the developing child during this time.

The embryo is approximately 1 inch in length and weighs about 4 grams at the end of this period. The embryo can move and respond to touch at this time. From the ninth week until birth, the organism is referred to as a fetus. By the time the fetus is considered full-term at 38 weeks gestation, he or she may be 50 centimeters 20 inches or 3. Although all of the organ systems were formed during embryonic development, they continue to develop and grow during the fetal stage.

Examples of some of the major features of fetal development by week are as follows:. Although 90 percent of babies born in the United States are considered healthy, abnormalities may arise during prenatal development that are considered congenital inherited or due to a genetic abnormality or environmental such as material derived abnormalities. In other cases, problems may arise when a fetus is born prematurely. In some cases abnormalities may arise during prenatal development that cause physical malformations or developmental delays or affect various parts of the body after the child is born.

The cause may be a small mutation in or damage to the genetic material of cells, or a major chromosomal abnormality each normal cell has two copies each of 23 strands [called chromosomes] of genetic material, and abnormalities can arise if there are three copies of a strand or only one.

Sometimes the abnormality is inherited from one or both parents; in other cases, the defect occurs because of an error in prenatal development. Some abnormalities are minor and do not affect the long-term prognosis once the child is born. At the other end of the spectrum, abnormalities may be so severe that fetal demise is inevitable.

Approximately 10 to 15 percent of pregnancies end before the twentieth week, a process called miscarriage or spontaneous abortion; congenital abnormalities account for a significant proportion of miscarriages.

Genetic abnormalities account for approximately 5 percent of miscarriages. The age, health status, nutritional status, and environment of the mother are all closely tied to the health of a growing embryo or fetus. Some examples of environmental factors that may lead to developmental abnormalities include:. Advances in medical care have made it possible for many infants born prematurely to survive and develop normally.

The earlier the gestational age, the greater the chance of death or significant medical problems. Whether or not a premature infant will survive is intimately tied to his or her gestational age:. Miscarriage —Loss of the embryo or fetus and other products of pregnancy before the twentieth week.

For this reason, miscarriage is also referred to as spontaneous abortion. Ovary —One of the two almond-shaped glands in the female reproductive system responsible for producing eggs and the sex hormones estrogen and progesterone. Teratogen —Any drug, chemical, maternal disease, or exposure that can cause physical or functional defects in an exposed embryo or fetus.

Uterus —The female reproductive organ that contains and nourishes a fetus from implantation until birth. Also called the womb. Many parents have questions or concerns about the prenatal development of an existing or anticipated child and what steps they should take to ensure their child's health. During prenatal visits to an obstetrician, a pregnant mother should be educated in proper nutrition and prenatal care; often, prenatal vitamins are prescribed to avoid nutritional deficiencies.

Prenatal testing is often recommended to parents-to-be as a means of assessing the fetus's health and the risk of developing certain conditions. Some common prenatal tests that relate to prenatal development are as follows:. Gilbert, Scott F.

Developmental Biology , 6th ed. Miller, Sharon M. March of Dimes Birth Defects Foundation. Hill, Mark. UNSW Embryology , In the following weeks, the fetus will develop hair, nails, teeth and the excretory and digestive systems will continue to develop.

At the end of the 12th week, the fetus is about 3 inches long and weighs about 28 grams. At about 16 weeks, the fetus is approximately 4.

Fingers and toes are fully developed, and fingerprints are visible. During the th months, the eyes become more sensitive to light and hearing develops.

The respiratory system continues to develop. Reflexes such as sucking, swallowing, and hiccupping develop during the 5th month. Cycles of sleep and wakefulness are present at that time as well. Throughout the fetal stage, the brain continues to grow and develop, nearly doubling in size from weeks 16 to The majority of the neurons in the brain have developed by 24 weeks although they are still rudimentary and the glial or nurse cells that support neurons continue to grow.

At 24 weeks the fetus can feel pain. The first chance of survival outside the womb, known as the age of viability is reached at about 22 to 26 weeks. The hearing has developed, so the fetus can respond to sounds. Between the 7th and 9th months, the fetus is primarily preparing for birth. It is exercising its muscles, its lungs begin to expand and contract. It is developing fat layers under the skin. The fetus gains about 5 pounds and 7 inches during this last trimester of pregnancy which includes a layer of fat gained during the 8th month.

This layer of fat serves as insulation and helps the baby regulate body temperature after birth. Around 36 weeks, the fetus is almost ready for birth. It weighs about 6 pounds and is about The fetus continues to gain weight and grow in length until approximately 40 weeks.

By then, the fetus has very little room to move around and birth becomes imminent. This video on prenatal development explains many of the developmental milestones and changes that happen during each month of development for the embryo and fetus.

Good prenatal care is essential. The developing embryo is most at risk for some of the most severe problems during the first three months of development. Unfortunately, this is a time at which most women are unaware that they are pregnant. Teratogens are factors that can contribute to birth defects which include some maternal diseases, drugs, alcohol, and stress.

These exposures can also include environmental and occupational exposures. Today, we know many of the factors that can jeopardize the health of the developing embryo and fetus. Some teratogen-caused birth defects are potentially preventable. The study of factors that contribute to birth defects is called teratology. Teratogens are usually discovered after an increased prevalence of a particular birth defect.

Exposure of the fetus during this early stage of development resulted in cases of phocomelia, a congenital malformation in which the hands and feet are attached to abbreviated arms and legs. One of the most commonly used teratogens is alcohol. Because half of all pregnancies in the United States are unplanned, it is recommended that women of child-bearing age take great caution against drinking alcohol when not using birth control and when pregnant.

There is no acceptable safe limit for alcohol use during pregnancy, but binge drinking 5 or more drinks on a single occasion or having 7 or more drinks during a single week places an embryo and fetus at particularly high risk. In extreme cases, alcohol consumption can lead to fetal death, but more frequently it can result in fetal alcohol spectrum disorders FASD.

This terminology is now used when looking at the effects of exposure and replaces the term fetal alcohol syndrome. It is preferred because it recognizes that symptoms occur on a spectrum and that all individuals do not have the same characteristics.

Children with FASD share certain physical features such as flattened noses, small eye openings, small heads, intellectual developmental delays, and behavioral problems. Those with FASD are more at risk for lifelong problems such as criminal behavior, psychiatric problems, and unemployment. ARBD include kidney, bone and heart problems. Several medical experts debunk common myths about the safety of drinking alcohol during pregnancy.

Last is the fetal period , which is the stage of prenatal development that occurs from the ninth week after conception until the fetus is born. This period consists of the fetus gaining approximately 7 pounds more than 3, grams and the organs continuing to develop, slowly gaining the ability to function independently. In the fetal stage, the developing human organism is called a fetus.

This is the longest stage, starting in the ninth week after conception and continuing until birth. During this stage, pregnant individuals often have an ultrasound done, which is an image made using high frequency sound waves that can show a fetus or an internal organ also called sonogram Goncalves, Source: Goncalves, L. Three-dimensional ultrasound of the fetus: How does it help? Pediatric Radiology, 46, — Sonographic markers of fetal a-thalassemia major. Journal of Ultrasound Medicine, 34,—