You: Having a Baby: The Owner’s Manual to a Happy and Healthy Pregnancy. Michael Roizen F.
body responds by producing antibodies that fight the positive antigens. Because there is no blood-to-blood contact during pregnancy, Rh disease is rare in a first pregnancy. But once mom’s antibodies have been activated, they will freely cross the placenta into the fetus’s bloodstream during subsequent pregnancies and tag the baby’s red blood cells for destruction, causing anemia or other serious conditions.
This situation can be prevented by giving an Rh-negative mother a prophylactic injection of an Rh immunoglobulin called RhoGam, which prevents the antibodies from forming, either midway during her first pregnancy or within seventy-two hours after delivery if it is known that the father is Rh-positive. In situations where the father’s blood type is unknown, it’s best for an Rh-negative woman to get the injection anyway to decrease the risk of this problem occurring. In circumstances where the mother suffers a miscarriage or has an ectopic pregnancy, it’s common practice to give her the injection within seventy-two hours if she is Rh-negative, to avert the chances of her forming antibodies due to lack of information about the father’s blood type in an emergency situation. Rh tests are given during prenatal visits to help ID potential problems.
body shifts from rebel mode* to ally mode, as it nurtures, feeds, and makes peace with what it should perceive as a foreign invader: the fetus.
Remember that 50 percent of the fetus’s genes are from the father and theoretically
The Down Low on CVS
No, it’s not an exposé on the drugstore chain; we’re talking about the diagnostic test called chorionic villus sampling (CVS). Here’s how it works: Because the placenta is made up of cells that derive from the same fertilized egg as the fetus, it’s an ideal resource for DNA that can reveal any genetic abnormalities in your developing baby (or for a paternity test). Chorionic villus sampling, performed between the tenth and twelfth weeks of pregnancy, entails removing a tiny sample of cells from the placenta. The specimen is then cultured in a laboratory, and the DNA is examined for genetic content.
Because CVS is an invasive test and carries a very small (less than 1 percent) chance of miscarriage, it’s generally performed when there’s an indication of risk: if you’ve had a prior pregnancy that involved a genetic problem, if there’s a history of genetic disease in either your family or the baby’s father’s, if you had an abnormal first-trimester screen or nuchal translucency test (see page 303), or if you will be thirty-five or older (although some do not follow the age indication) at the time of delivery and therefore at higher risk of having a baby with Down syndrome. While CVS can detect up to two hundred different genetic defects with 98 percent to 99 percent accuracy (including Tay-Sachs disease, cystic fibrosis, and hemophilia), it cannot detect neural tube defects such as spina bifida (more on page 112), which can be picked up by amniocentesis. The advantage of CVS over amniocentesis is that you can have the test done much earlier, as amnio is usually performed between sixteen and twenty weeks.
To prepare for your CVS, drink heavily beforehand (water, please), because you’ll need a full bladder. Your doctor will first perform an ultrasound to assess the baby’s position and the orientation of your uterus to determine whether she’ll do the CVS vaginally, threading a catheter through your cervix, or abdominally, via needle biopsy. In either case, you might experience slight cramping during and after the procedure, as well as light spotting if you had it done vaginally. You should plan to have someone drive you home and you’ll need to take it easy for the rest of the day. By the following day, you should feel fine; if you experience increased bleeding, vaginal discharge, or fever, call your doctor immediately. Some docs will have you come in a few days after the procedure for a follow-up ultrasound just to make sure everything is okay. Results will be ready in one to two weeks.
Take a Look
In the pecking order of things you want to look at in the delivery room, your placenta may rank pretty close to the bottom of the list. But that doesn’t mean your doc or midwife should feel the same way. While you’re gazing at your little darlin’, your provider should be inspecting your cute, adorable, squishy organ. Why? A one-minute examination of the placenta provides info that may be important to the care of both mother and infant. The doctor will assess the size, shape, and consistency of the placenta, as well as the umbilical cord. She’ll also make sure it’s complete and has no missing parts, for leaving some of the tissue in the body can cause hemorrhages up to two weeks later. The delivered placenta can also give clues to future problems, and if something looks abnormal, tests can help determine the best course of treatment or monitoring in future pregnancies. A thin cord can indicate a stressed environment in utero, which may help you make different lifestyle decisions during your next pregnancy. And a placenta that has calcification may indicate a decreased delivery of nutrients and that mom may have a condition that has compromised her arterial health, such as unrecognized changes in blood pressure.
can carry information that could sabotage the relationship between mother and child. But the mother’s immune system overlooks this fact and seeks to protect the child anyway. In a way, you can consider pregnancy an immunosuppressed state; the volume of mom’s immune system is turned down as it deals with this foreign, yet welcome, invader. As we mentioned, the most dangerous time for the newly created creature is prior to implantation in the uterine wall. If mom’s immune system is not suppressed when the fertilized egg tries to land, it’s bye-bye, blastocyst. So exactly one day before implantation—six days after conception—the blastocyst produces a special enzyme that suppresses mom’s killer T cells, preventing her from having an immune reaction to the baby-to-be’s cells. Once the placenta is formed after implantation, it helps maintain the truce between these two potential adversaries.
A baby’s immune system isn’t fully developed before birth, so it needs a hand from mom, who passes her own immune-system warriors, called antibodies, through the
Immunity Granted
As we get older, we tend to think of lymph nodes only when they get sore or swollen, but they actually play an important role in fetal immunity. After T cells mature in the thymus, they engage the enemy on the biological battleground: the five hundred to six hundred peripheral lymph nodes distributed throughout the body. Once T cells have recognized the enemy, they head into the spleen, where B cells are made. The thymus doesn’t change its responsiveness after birth, but the lymph nodes and spleen change big-time. In the fetus, about a quarter of the T cells in the lymph nodes and spleen are regulatory, meaning that their job is to prevent a child’s immune system from overresponding.
Why would this happen? Immune cells from the mother that slip through the placenta into the child’s body will be sequestered in the lymph nodes. After all, that’s where infections collect as you grow up and overcome, for example, a sore throat and get large, tender lymph nodes in the local neck area. By not overresponding to the mother’s immune cells, the fetus develops tolerance. In other words, the mother’s immune cells are instructing the fetal cells to tolerate their presence. This parental guidance might not work in teenagers but works like a charm in the womb. These insights are helping scientists figure out how to perform safer transplantations so that patients do not reject the donated organs.
placenta. This is called passive immunity because the fetus doesn’t make antibodies itself but accepts the mother’s. Interestingly, it even turns out that some cells from mom slip through the placenta to teach the baby’s cells how to tolerate foreign antigens—diplomatic cells, if you will.
This transmission of antibodies serves two roles: protecting the fetus and signaling to the mother that this foreign tissue has cleared inspection and can be safely allowed to park its behind in her uterus for the greater part of the next year. Now, a fetus’s immune system doesn’t start developing on its own until nine weeks and isn’t up and running until week fourteen. The reason for that delay is to help the fetus to tolerate mom—after all, the fetus is 50 percent dad. If its immunity was fully developed from the get-go, there’d be the chance that the fetus might identify mom as different