Second trimester prenatal screening may include several blood tests, called multiple markers. These markers provide information about a woman's risk of having a baby with certain genetic conditions or birth defects. Screening is usually performed by taking a sample of the mother's blood between the 15th and 20th weeks of pregnancy (16th to 18th is ideal). The multiple markers include:
- An alpha-fetoprotein (AFP) blood test checks the level of AFP in a pregnant woman's blood. AFP is a substance made in the liver of an unborn baby. The amount of AFP in the blood of a pregnant woman can help see whether the baby may have such problems as spina bifida and anencephaly.
- Chorionic villus sampling (CVS) is a prenatal test that involves taking a sample of some of the placental tissue. This tissue contains the same genetic material as the fetus and can be tested for chromosomal abnormalities and some other genetic problems.
- An amniocentesis is a procedure used to obtain a small sample of the amniotic fluid that surrounds the fetus to diagnose chromosomal disorders and open neural tube defects (ONTDs) such as spina bifida. There is a small risk of miscarriage associated with amniocentesis which must be balanced with the risk of an abnormality and the patient’s desires.
Week 17: Your baby, now about in its 15th week of development, measures about 4.4 to 4.8 inches from crown to rump and has doubled in weight in the last two weeks to about 3.5 ounces. Fat begins to form, helping your baby's heat production and metabolism. The lungs are beginning to exhale amniotic fluid, and the circulatory and urinary systems are working. Hair on head, eyebrows and eyelashes is filling in.
Week 18: Your baby measures 5 to 5.6 inches from crown to rump and weighs about 5.25 ounces. The rapid growth spurt is tapering off, but reflexes are kicking in. It can yawn, stretch and make facial expressions, even frown. Taste buds are beginning to develop and can distinguish sweet from bitter tastes. The baby will suck if its lips are stroked and it can swallow, and even get the hiccups. The retinas have become sensitive to light, so if a bright light is shined on your abdomen, baby will probably move to shield its eyes.
Week 19: Your baby measures about 5.2 to 6 inches from crown to rump and weighs about 7 ounces. Skin is developing and transparent, appearing red because blood vessels are visible through it. Creamy white protective coating, called vernix, begins to develop.
Week 20: The fetus measures about 5.6 to 6.4 inches from crown to rump and weighs about 9 ounces. Your baby can hear sounds by now -- your voice, heart and your stomach growling, as well as sounds outside your body. It will cover its ears with its hands if a loud sound is made near you, and it may even become startled and "jump." The baby is moving often, too -- twisting, turning, wiggling, punching and kicking.
Week 21: Your baby measures about 7.2 inches from crown to rump and weighs about 10.5 ounces. The fetus is steadily gaining fat to keep warm. Growth rate is slowing down but organ systems, like digestion, are continuing to mature. A waxy film, called the vernix caseosa, is being produced by your baby's oil glands and covers the skin to keep it supple in the amniotic fluid. Buds for permanent teeth are beginning to form.
Week 22: Your baby measures about 7.6 inches and weighs about 12.3 ounces. The muscles are getting stronger every week now, and the eyelids and eyebrows are developed. Your baby's acrobatics are pretty constant, and since he responds to sound, rhythm and melody, you can try singing and talking to him. After he's born, the same sounds will soothe him.
Week 23: Your baby is about 8 inches from crown to rump and weighs almost 1 pound. The body is becoming proportioned more like a newborn, but skin is still wrinkled because your baby still has more weight to gain. Lanugo hair on the body sometimes turns darker.
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