Continuing our series on the biology of pregnancy, let’s move into the middle weeks of pregnancy, starting with gestational week 18. At this point, the external ears of the fetus are fully formed and they have moved to their final destination on the head. Ossification, hardening of the bones, is in full swing and will continue throughout the fetal period and after birth. In mid-pregnancy, however, there is noticeable ossification in the leg bones and the three tiny bones of each middle ear. Teeth are growing and hardening under the gums, plus the fetus is beginning to excrete meconium, an early kind of feces, from the anus, into the amniotic fluid. Extraction of a sample of amniotic fluid for testing is called amniocentesis, a procedure that is still possible during this part of pregnancy and which can reveal chromosomal, and neural tube defects. In contrast, chorionic villus sampling (CVS) is performed somewhat earlier in pregnancy. Recall that the neural tube is a long segment of tissue that has folded into a tube during the embryonic period and gives rise to the central nervous system –the brain and spinal cord. But there can be imperfections in the closure, resulting in part of the brain or spinal cord uncovered by bone and other tissues that normally surround it. One example is spina bifida, absence of bone around parts of the spinal cord. Another neural tube defect is anencephaly, where part of the skull is missing, along with most of the brain. If you’re an older mother, which in obstetrics means above 35 years, you’re a candidate for amniocentesis. You’re also a candidate if your doctor determines that you carry a risk for a neural tube defect.
By this point of pregnancy ultrasonography is reliable to tell you the gender of your fetus. If you have a girl, she has about 6 million eggs (ova) in her developing ovaries! This number will decrease substantially by birth time and will continue to drop, but one or more of those millions of those eggs can give you grandchildren. Yes, you are already an ancestor. As for the fetal uterus and fallopian tubes, those are developing too. If your fetus is male, development of the prostate gland is moving forward.
Meanwhile, the fetus is moving a lot. He or she will tumble and change position several times between now and the time of birth. As the fetal heart grows and gets stronger, the cardiac output (the volume of blood pumped per minute) will increase, but the heartbeat has been slowing gradually for the past eight weeks or so. By this point, it has dropped below 150 beats per minute on average and the heart rate will continue to fall.
By week 19, development of the hair-like substance, lanugo, is at a peak. The lanugo is thus able to hold onto a cheesy-looking coating called the vernix caseosa. It consists of dead skin cells and oil secreted from glands, and it protects the skin from wrinkling and chapping. The skin also is producing melanocytes, cells that later will protect the skin from intense sunlight. Some hair may grow in the scalp at this point, plus the volume of urine excreted continues to increase, so the volume of amniotic fluid increases tool. Meanwhile, the fetal kicks —quickening— become more powerful and noticeable, because the legs are growing and their muscles are getting stronger.
When you enter week 20, officially, you are halfway through pregnancy, but not technically speaking, since pregnancy didn’t really begin until the end of week 3. The fetus at this point is approximately 16.5 centimeters long, about the length of a banana. Underlying this, growth hormone from the tiny pituitary gland, located at the base of the brain, reaches its peak. From this point, the level of growth hormone will drop.
If you haven’t had a triple test, you can have one at this routine exam at this point. A triple test checks levels of three substances: alpha fetoprotein (AFP), estriol, and beta-hCG. This can tell your obstetrician if your fetus is at risk for certain developmental anomalies. Addition of a test for levels of inhibin A make the test a quadruple test. Everybody should have such testing, but it’s especially important if you are over 35 year of age, have diabetes, have used certain medications that may cause birth defect, have been exposed to extremely high levels of ionizing radiation, have suffered a viral infection early in pregnancy, or if birth defects run in your family. Depending on how the testing comes out, you may require a very detailed ultrasound. Fat is continuing to deposit beneath the fetal skin, and it’s mostly a special kind of fat called brown fat. Unlike white fat, brown fat can actually generate heat, so it can help keep a newborn warm while also adding some cushioning for protection.
Hormones will continue to make you sweat more than you normally do, plus the vaginal discharge will continue and you may experience some reddening on your skin. Your belly seems to be popping out more with each passing day, plus your breasts are getting, and feeling, more and more engorged. Along with the usual bloating, other things that may bother you at this point are headaches. These may be due to sleep difficulty, but bright lights in the morning and dim lighting at night can help mitigate this problem. Also you may experience swelling, but drinking a lot of water can help with this.