If you could see your baby right now, you'd laugh as she tests out her developing muscles by making lots of funny faces—squinting, frowning, even smiling in the womb. You'd be astonished by her paper-thin skin, through which a network of fine blood vessels runs like a series of miniature roads. Beneath her skin, your baby's skeleton is hardening, transforming from cartilage to bone as she continues to absorb calcium. Her bones will stay flexible, however, so that she can easily make the journey through your birth canal. They won't harden enough for her to stand until she's a toddler.
Her ears are nearly in position, and she can turn her head. She can make fists and point her toes too, though most of the time these movements are reflexive. Her hair has started to grow, and it may have a little color as the pigment cells of her hair follicles kick in. You'll need to decide soon whether you want to know the sex of your baby before you give birth; the external sex organs are nearly complete, so an ultrasound can reveal whether you're having a boy or a girl.
An amniocentesis is performed to obtain a sample of the amniotic fluid surrounding a baby. To obtain the sample, the physician will insert a long needle through the abdominal wall into a pocket of amniotic fluid in the uterine cavity. From that fluid pocket, a small amount of amniotic fluid is withdrawn through the needle. The fluid is sent to a laboratory for analysis of the chromosomal patterns of cells your baby has shed into the fluid as a part of the baby's growth cycle. Through analysis of the baby's cells found in the fluid, physicians can get a detailed description of your baby's chromosomes. Results from an amniocentesis carry a 99.4 percent accuracy rate.
Patients generally feel very little discomfort during amniocentesis procedures. Physicians often use ultrasound imaging to help guide the needle away from the baby, the umbilical cord, and the center of the placenta. But an amniocentesis is not without risks. In a small number of pregnant women, an amniocentesis can result in complications, including the loss of the pregnancy. Any pregnant woman may request an amniocentesis, but patients considering the procedure should discuss the risks and benefits of the procedure with a physician or genetic counselor. Another highly accurate prenatal chromosomal test is called Chorionic Villus Sampling (CVS). Similar to the amniocentesis, the physician uses ultrasound imaging as a guide while performing the procedure. For a CVS, a thin, plastic tube is inserted into the abdomen or through the vagina in order to collect a small sampling of placental tissue. An advantage to CVS over amniocentesis is that the procedure can be performed earlier in a pregnancy, around 10 weeks. But, CVS does carry a slightly higher risk of complications in comparison to an amniocentesis.
Neither procedure tests for every genetic abnormality.
Amniocentesis: An invasive procedure performed by a physician to collect a sample of amniotic fluid from a developing pregnancy to check for fetal genetic abnormalities. Ideally performed between 15 and 18 weeks.
Chorionic villus sampling (CVS): An invasive procedure performed by a physician to collect a sample of placental tissue from a developing pregnancy to look for fetal genetic abnormalities. Can be performed as early as 10 weeks.
Images courtesy of the American Institute of Ultrasound in Medicine (AIUM.org).