Week 16: The Quad Screen

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This is your Pea in the Podcast for week 16 of your pregnancy. I'm Bonnie Petrie joined by Dr. Laurie Swaim, an obstetrician with Houston Women's Care Associates in Houston, Texas.

It's time to talk about another test. This time just a blood test, between weeks 15 and 21 but most often between week 16 and 18 when it's most accurate, you'll be offered the quad screen, "The quad screen is looking for, once again, the baby's risk for Down syndrome and neural tube defects. These neural tube defects were not included in the first trimester screening. If a woman had the first trimester screen then she needs either this test or one fourth of it called the Maternal Serum Alpha-Fetoprotein Test (MSAFP). This is just a blood test, it will also come back with a numeric value that will tell the patient her risk based on her age. So a pretest and then the post test risk of Down syndrome and post test risk of spina bifida."

Okay so the quad screen measures four proteins, the pattern of these proteins in combination with your number of weeks in your pregnancy, your race, your age, your weight and whether you're an insulin dependent diabetic will give you a sense of your baby's risk for neural tube defects, Down syndrome and trisomy 18. But Dr. Swaim says the quad screen makes some women nervous because there is a risk of getting what is sometimes called a false positive, "the false positive rate is upwards of 8% on this test." Why will so many women who consent to this blood test get unnecessarily worrisome results, "Because the test is not the most sensitive in the world, in order to make it to have its highest sensitivity then we accept the fact that it identifies some normal babies as being abnormal so we'll try and catch all the abnormal ones." So the quad screen casts a wide net and rather than missing babies who might have chromosomal abnormalities, it sometimes flags babies who don't. So, if your quad screen comes back outside the normal range it is no reason to panic, it's just time for a little more investigation, "This test is very dependent on dates, how many weeks pregnant people are, so you want to make sure that their dates are actually correct. So the first thing you want to make sure if she's done the test at 16 weeks, she really is at 16 weeks. Another thing to make sure is that there are no twins or triplets because that can throw off the test also. So the first thing usually she's sent for a targeted ultrasound and she's offered an amniocentesis if she desires because I tell my patients you go right where the money is and you look and you see, does the baby have a chromosomal problem or doesn't it. Some women opt to have the amnio and some don't, some are satisfied with a normal ultrasound. Now I'll tell you also though that certain parts of the anatomy cannot be evaluated as well as possible at 16 weeks so most often these women will be asked to come back for the normal 20 week anatomy ultrasound and that's to look at the cardiac anatomy specifically."

So you and your doc will talk about more testing. Sometimes when the results are outside the normal range but no chromosomal arrangement problem is detected you will still be watched more closely. "Sometimes there's an unexplained elevated MSAFP, now if it's Down syndrome we're actually more concerned that it might be low. Sometimes the test comes back abnormal, the baby is structurally normal, the chromosomes are normal, but the MSAFP is high and in those babies we are concerned with the possible development of growth restriction or other kind of consequences later on in the third trimester." So they'll watch you more closely. Now as you get older the chance of an abnormal quad screen will increase because age is a factor included in that formula. Also the quad screen is not a definitive test. As we mentioned a positive screen does not mean that your baby has a chromosomal abnormality or spina bifida, it just means you might want to consider more testing. If it's negative, well according to the American College of Obstetricians and Gynecologists on average the quad screen detects Down syndrome in 81% of the cases. The AFP portion of the test detects neural tube defects in 80% of the cases. So should you consider this test? "I offer Down syndrome screening and neural tube defects screening to every patient and so for women who didn't have a first trimester screen, we'll just offer them the quad screen if they want. And some have told us earlier on, they want no testing so I don't bother them about it necessarily." The ultimate decision on this, as all prenatal testing, rests with you and your doctor.

So what else is going on now that you're 16 weeks pregnant? Well your baby's a dancing fool in there even though you still can't probably feel it. They may suck on their fingers or toes or play with their umbilical cord. Those eyes that started out on the sides of their head have migrated toward the front, their almost where they belong and they're moving under those eyelids. Your baby will hold their head more erect now and their neck is quite developed. If you have a baby girl in there she's busy getting ready to make you a grandma, millions of eggs are forming in her tiny ovaries. Your baby weighs around three ounces and is about four inches long this week.

So what about you? Well there's not a heck of a lot new going on. Stuffy noses and nose bleed may still annoy you. Round ligament pain may still take your breath away as your uterus grows, some moms will notice some swelling in their feet already, it is normal although not a lot of fun. When you notice your feet swelling, try to put them up and as always if you're concerned give your doctor or midwife a call.

You're 16 weeks pregnant, you have 24 weeks to go until the end of the line, week 40.

That is your Pea in the Podcast for week 16. Dr. Swaim and I look forward to talking to you again next week. Enjoy this week. And for a transcript of any of our Pea in the Podcasts go to our website peainthepodcast.com. For Pea in the Podcast, I'm Bonnie Petrie, thanks for listening.