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Introduction to Fetal Period
In this lecture educator explains the fetal period. Development from embryo to fetus is not abrupt, but the embryo changes to a recognizable human being and develops all the basic outlines of its organs and is then called a fetus. This long (7-month) fetal period is concerned with growth and differentiation of tissues and organs that began to develop in the embryonic period, maturation of the primordia, reorganization of spatial relationships of primordia, and the embryo begins to make functional use of its organs for part of its needs. Its volume and weight increase proportionally, and it grows considerably, from about 30 mm to about 330 mm. Fetal growth is complex and is really a phenomenon which results from the sum of very asynchronous growth of different organs and parts even at the histologic level (histogenesis). Body proportions at term are very different from the fetus of 2 or 3 months.
Crown-rump length (CRL) is the measurement of the length of human embryos and fetuses from the top of the head (crown) to the bottom of the buttocks (rump). It is typically determined from ultrasound imagery and can be used to estimate gestational age.
Monthly Changes in Developing Fetusus
Month 1: As the fertilized egg grows, a water-tight sac forms around it, gradually filling with fluid. This is called the amniotic sac, and it helps cushion the growing embryo. The placenta also develops. The placenta is a round, flat organ that transfers nutrients from the mother to the baby, and transfers wastes from the baby. A primitive face will take form with large dark circles for eyes. The mouth, lower jaw, and throat are developing. Blood cells are taking shape, and circulation will begin. The tiny "heart" tube will beat 65 times a minute by the end of the fourth week. By the end of the first month, baby is about 1/4-inch-long, smaller than a grain of rice.
Month 2: Baby's facial features continue to develop. Each ear begins as a little fold of skin at the side of the head. Tiny buds that eventually grow into arms and legs are forming. Fingers, toes and eyes are also forming. The neural tube (brain, spinal cord and other neural tissue of the central nervous system) is well formed. The digestive tract and sensory organs begin to develop. Bone starts to replace cartilage. The head is large in proportion to the rest of the baby's body. By the end of the second month, baby is about 1 inch long and weighs about 1/30 of an ounce. At about 6 weeks, baby's heart beat can usually be detected. After the 8th week, baby is called a fetus instead of an embryo.
Month 3: Baby's arms, hands, fingers, feet, and toes are fully formed. Baby can open and close its fists and mouth. Fingernails and toenails are beginning to develop, and the external ears are formed. The beginnings of teeth are forming. Baby's reproductive organs also develop, but the baby's gender is difficult to distinguish on ultrasound. By the end of the third month, baby is fully formed. All the organs and extremities are present and will continue to mature in order to become functional. The circulatory and urinary systems are working, and the liver produces bile. At the end of the third month, baby is about 4 inches long and weighs about 1 ounce. Since baby’s most critical development has taken place, chance of miscarriage drops considerably after three months.
Month 4: Baby's heartbeat may now be audible through an instrument called a doppler. The fingers and toes are well-defined. Eyelids, eyebrows, eyelashes, nails, and hair are also formed. Teeth and bones become denser. Baby can even suck his or her thumb, yawn, stretch, and make faces. The nervous system is starting to function. The reproductive organs and genitalia are now fully developed, and doctor can see on ultrasound if are having a boy or a girl. By the end of the fourth month, baby is about 6 inches long and weighs about 4 ounces.
Month 5: Hair begins to grow on baby's head. Baby's shoulders, back, and temples are covered by a soft fine hair called lanugo. This hair protects baby and is usually shed at the end of the baby's first week of life. The baby's skin is covered with a whitish coating called vernix caseosa. This "cheesy" substance is thought to protect baby's skin from the long exposure to the amniotic fluid. This coating is shed just before birth. By the end of the fifth month, baby is about 10 inches long and weighs from 1/2 to 1 pound.
Month 6: Baby's skin is reddish in color, wrinkled, and veins are visible through the baby's translucent skin. Baby's finger and toe prints are visible. The eyelids begin to part and the eyes open. Baby responds to sounds by moving or increasing the pulse.
Month 7: Baby will continue to mature and develop reserves of body fat. Baby's hearing is fully developed. He or she changes position frequently and responds to stimuli, including sound, pain, and light. The amniotic fluid begins to diminish. At the end of the seventh month, baby is about 14 inches long and weighs from 2 to 4 pounds. If born prematurely, baby would be likely to survive after the seventh month.
Month 8: Baby will continue to mature and develop reserves of body fat. The baby kicks more during this month. Baby's brain is developing rapidly at this time, and baby can see and hear. Most internal systems are well developed, but the lungs may still be immature. Baby is about 18 inches long and weighs as much as 5 pounds.
Month 9: Baby continues to grow and mature. The lungs are nearly fully developed. Baby's reflexes are coordinated so he or she can blink, close the eyes, turn the head, grasp firmly, and respond to sounds, light, and touch. Baby is definitely ready to enter the world. The baby moves less due to tight space. Baby's position changes to prepare itself for labor and delivery. The baby drops down in pelvis. Usually, the baby's head is down toward the birth canal. Baby is about 18 to 20 inches long and weighs about 7 pounds.
Estimation of Embryonic and Fetal Age
Growth rate of fetus is linear up to 37 weeks of gestation, after which it plateaus. The growth rate of an embryo and infant can be reflected as the weight per gestational age, and is often given as the weight put in relation to what would be expected by the gestational age. A baby born within the normal range of weight for that gestational age is known as appropriate for gestational age (AGA). An abnormally slow growth rate results in the infant being small for gestational age, and, on the other hand, an abnormally large growth rate results in the infant being large for gestational age. A slow growth rate and preterm birth are the two factors that can cause a low birth weight. Low birth weight (below 2000 grams) can ultimately increase the likelihood of schizophrenia by almost four times.
The growth rate can be roughly correlated with the fundal height which can be estimated by abdominal palpation. More exact measurements can be performed with obstetric ultrasonography.
Time of Birth
The date of birth is most accurately indicated as 266 days, or 38 weeks, after fertilization. The oocyte is usually fertilized within 12 hours of ovulation; however, sperm deposited in the reproductive tract up to 6 days prior to ovulation can survive to fertilize oocytes. The obstetrician calculates the date of birth as 280 days or 40 weeks from the ﬁrst day of the LNMP. In women with regular 28-day menstrual periods, the method is fairly accurate, but when cycles are irregular, substantial miscalculations may be made. Most fetuses are born within 10 to 14 days of the calculated delivery date. If they are born much earlier, they are categorized as premature and if born later, they are considered post mature. By combining data on the onset of the last menstrual period with fetal length, weight, and other morphological characteristics typical for a given month of development, a reasonable estimate of the age of the fetus can be formulated.
Assessment Through Ultrasound
- A valuable tool for assisting in this determination is ultrasound, which can provide an accurate (1 to 2 days) measurement of CRL during the 7th to 14th weeks.
- Measurements commonly used in the 16th to 30th weeks are bi parietal diameter (BPD), head and abdominal circumference, and femur length.
- An accurate determination of fetal size and age is important for managing pregnancy, especially if the mother has a small pelvis or if the baby has a birth defect.
Low birth weight
Low birth weight infants are at increased risk of renal disease. A reduced complement of nephrons at the beginning of life in infants that are intrauterine growth restricted (IUGR) and/or born preterm may be the cause of their long-term risk of renal disease. A reduced nephron endowment in low birth weight infants may also lead to vulnerability to hypertension in adulthood; however, experimental evidence suggests that they are not causally linked.
Intrauterine growth restriction(IUGR) refers to a condition in which an unborn baby is smaller than it should be because it is not growing at a normal rate inside the womb.
Delayed growth puts the baby at risk of certain health problems during pregnancy, delivery, and after birth. They include:
- Low birth weight
- Difficulty handling the stresses of vaginal delivery
- Decreased oxygen levels
- Hypoglycemia (low blood sugar)
- Low resistance to infection
- Low Apgar scores (a test given immediately after birth to evaluate the new born's physical condition and determine need for special medical care)
- Meconium aspiration (inhalation of stools passed while in the uterus), which can lead to breathing problems
- Trouble maintaining body temperature
- Abnormally high red blood cell count
In the most severe cases, IUGR can lead to stillbirth. It can also cause long-term growth problems.
Causes of Intrauterine Growth Restriction: IUGR has many possible causes. A common cause is a problem with the placenta. The placenta is the tissue that joins the mother and fetus, carrying oxygen and nutrients to the baby and permitting the release of waste products from the baby. The condition can also occur as the result of certain health problems in the mother, such as:
- Advanced diabetes
- High blood pressure or heart disease
- Infections such as rubella, cytomegalovirus, toxoplasmosis, and syphilis
- Kidney disease or lung disease
- Malnutrition or anemia
- Sickle cell anemia
- Smoking, drinking alcohol, or abusing drugs
- Other possible fetal causes include chromosomal defects in the baby or multiple gestation (twins, triplets, or more).
IUGR Symptoms: The main symptom of IUGR is a small for gestational age baby. Specifically, the baby's estimated weight is below the 10th percentile -- or less than that of 90% of babies of the same gestational age. Depending on the cause of IUGR, the baby may be small all over or look malnourished. They may be thin and pale and have loose, dry skin. The umbilical cord is often thin and dull instead of thick and shiny. Not all babies that are born small have IUGR. IUGR Diagnosis: Doctors have many ways to estimate the size of babies during pregnancy. One of the simplest and most common is measuring the distance from the mother's fundus (the top of the uterus) to the pubic bone. After the 20th week of pregnancy, the measure in centimetres usually corresponds with the number of weeks of pregnancy. A lower than expected measurement may indicate the baby is not growing as it should.