Your Burning Questions About Prenatal Testing, Answered
November 15, 2017
It sounds straightforward and reasonable enough: Over the course of your pregnancy, your doctor will perform a battery of prenatal tests to make sure you and baby are in good health and the pregnancy is progressing along nicely. But once the abstract becomes reality, the protocol can suddenly become confusing and the results downright indecipherable. What’s more, new testing alternatives seem to surface in the news all the time, and it’s hard to tell what’s worth exploring and what’s oversold—and even tougher to know what questions to ask your doctor during an appointment. To help, we gathered up some of the most-asked questions concerning prenatal testing; here’s what the experts had to say.
How do I know everything is okay in the early weeks of pregnancy without a scan?
It’s hard, so the best thing to do to protect your pregnancy is to start taking prenatal vitamins and refrain from alcohol. Blood work can confirm a pregnancy, but an ultrasound is really the only way to verify baby’s status. In most cities, that’s often performed at your first visit, around eight weeks. “An ultrasound confirms that the pregnancy is inside a uterus, that it’s a live baby and not a miscarriage and that you have a single baby and not twins,” explains Fahimeh Sasan, DO, assistant professor obstetrics, gynecology and reproductive science at the Icahn School of Medicine at Mount Sinai in New York City. While you won’t be able to see baby’s limbs and organs at this early stage, a scan can also estimate gestational age and check for fetal heart rate. In addition to your first prenatal visit, you’ll also likely get an ultrasound as part of a first-trimester screen between weeks 12 to 13, and then again at the 20-week anatomy scan.
Why would a doctor do a transvaginal ultrasound as opposed to an abdominal one?
Whether your doctor will use a transvaginal ultrasound or a transabdominal one largely depends on what he’d like to find out. “The transvaginal ultrasound is best for determining cervical length and to assess for placenta previa,” says Jeffrey Kuller, MD, a maternal-fetal medicine specialist and professor of obstetrics and gynecology at Duke University School of Medicine. In this case, you’ll undress from the waist down and, as with a pelvic exam, place your feet in stirrups. The transducer is shaped like a wand and lubricated before being inserted into the vagina. (Tip: Empty your bladder before the exam.) It’s not exactly comfortable, but take comfort in knowing that it’s rarely performed after the first trimester, and you can look forward to a transabdominal ultrasound, where the transducer is used on the surface of your belly.
Do I have to get a carrier screen?
No, it’s entirely up to you. All women who are pregnant or are planning to be pregnant are offered tests for cystic fibrosis, hemoglobinopathies and spinal muscular atrophy, but there are other disease you can test for, and if you have a family history of certain disorders you might consider going for it. Most of these diseases are recessive, so if by chance you tested positive for one, your partner will be tested as well. If the disease is extremely rare, then chances are your child will be unaffected (since both you and your partner would need to test positive for the trait to surface in your child). But “chances” are a personal thing. One couple might be fine with a 1 percent chance, and another might not be—which is why pre-test counseling is always a good idea, says Jennifer Hoskovec, MS, CGC, National Society of Genetic Counselors Prenatal Expert and director of prenatal genetic counseling services at McGovern Medical School in Houston.
What accounts for false positives in an abnormal genetic screen?
Genetic screens obtain results by looking at levels of certain proteins and hormones in the mother’s blood. If they fall at a level that’s higher or lower than what’s considered normal, then it’s labeled “abnormal.” But the truth is, “There’s normal variation in levels of these substances,” Kuller says, and there are outliers.
If a cell-free fetal DNA test is 99 percent accurate, why should I get a CVS or amniocentesis?
The fetal DNA test has a 99 percent detection rate for Down syndrome, which is a subtly different concept from “accuracy.” Also, that rate is lower for other diseases. That’s because the cell-free fetal DNA test is a screen, not a diagnostic test, so the results won’t come back as a definite “yes” or “no.” Instead, you’re given a risk assessment. So with the same “positive” result from the lab, a 40-year-old may have a 90 percent chance of having a child with Down syndrome, while a 25-year-old may have a 50 percent chance. “That’s why we don’t recommend making any irreversible decisions based on these tests,” Hoskovec says. An amniocentesis and a CVS are the only ways to get a straight answer; what’s more, they’ll assess all 46 chromosomes for genetic disorders.
What is fundal height, and if it’s off, is it something to be concerned about?
In this day and age of ultrasounds, fundal height is rarely used. It’s measured with an old-fashioned tape measure (think what the tailor uses), from the pubic bone to the top of the uterus. After 20 weeks, that length should correspond to the number of weeks you’ve been pregnant. “For a period of time it was the gold standard for knowing whether your baby is growing well,” Sasan says. But now, with an ultrasound, you can find baby’s exact size.
I was told I have placenta previa. Is a c-section inevitable?
Not necessarily. In fact, placenta previa usually resolves itself over the course of the pregnancy. And with more ultrasounds being done nowadays, the detection of the condition has increased, when in the past women may not have even realized they had it midpregnancy. In a 2011 American Journal of Perinatology paper, out of 366 cases of placental previa, 84 percent of complete placenta previas and 98 percent of marginal placenta previas resolved themselves by around 28 weeks. The chances of resolution depend on the gestational age and distance of the placenta from the cervical opening. Your doctor will follow the position of baby over the course of your pregnancy with ultrasounds, and she’ll recommend lifestyle adjustments (such as avoiding intense exercise or vaginal intercourse) to keep baby safe. Women with a partially covered cervix at the time of delivery may be able to give birth vaginally, but chances are a c-section may be the safest option.
Is there anything I should eat to get good results at the glucose test?
You can’t really “game” the glucose test. True, if you scarf down a large coffee cake 30 minutes before your test, then you’ll probably fail it. But if you eat normally and then refrain from eating or drinking for an hour before the test (which your doctor will ask you to do), Sasan says, the results will give an accurate assessment of your system’s ability to metabolize sugar.
My quad test came back positive. Should I be worried?
An important thing to remember is that a “positive” result isn’t what most people think it means, Hoskovec explains. A positive result simply means you have a higher chance of having a child with a particular disease than the cutoff, which in this case is 1 in 270—meaning that in a population of 270 women in your same exact circumstances, there will be one with a child with Down syndrome. If you have a lower chance than these odds, then you end up with a negative result. One in 270 was chosen because it’s equal to the risk of miscarriage with an amnio (at least back when the numbers were established). “It’s kind of arbitrary,” says Hoskovec, “but it provides some sort of comparison when you’re trying to figure out if you should get further testing.” Your risks take age into account, which means that a 40-year-old and 25-year-old woman could have the same readings in terms of the blood test, but the result can come back “positive” for the older woman and “negative” for the younger. In addition, the same risk number could be worrisome to some women and not to others. By talking with a genetic counselor, you can make an informed decision about next steps, whether it’s to proceed with cell-free fetal DNA testing, an amniocentesis or nothing at all.
What is a soft marker?
Soft markers are findings that are likely normal variations in development but have a slight association with chromosomal disorders, such as Down syndrome. For instance, more babies with Down syndrome are found to have an echogenic intracardiac focus, a small bright spot seen in the heart on an ultrasound. However, it’s also seen in 5 percent of all pregnancies as well as in many Asian fetuses. “It’s just one piece of the puzzle,” Hoskovec says. “We see how they fit into the big picture. If it’s the only thing we see and the woman is under 35 and the results of the other screenings came back normal, then we’d be less than concerned. But if the patient were older than 35 and had other screening results suggesting higher risk, then we’d be more concerned.”
I’m past my due date and my doctor just ordered an antepartum test. What does that mean?
Doctors order the test simply to make sure everything is okay in there. It’s noninvasive and won’t hurt a bit. It’s done in the third trimester for high-risk pregnancies and pregnancies that go past the expected due date. Essentially, antepartum testing is a biophysical profile, which consists of an ultrasound that checks out the baby’s body and breathing movements, muscle tone and amniotic fluid, as well as a nonstress test, which monitors baby’s heart rate. Each observation receives a score of 0 (abnormal) or 2 (normal) and are then added together. If the total, out of 10, is less than 6, then the doctor might do more tests or take measures to deliver the baby early. If the score is above six, you’ll repeat the test once or twice a week until baby arrives. (A score of six is borderline and can go either way.)
Published November 2017