Antenatal Care: Screening for Down Syndrome (Aneuploidy)

00:00 Hi, now we're going to discuss screening for Down syndrome.

00:07 So it's three ways that we screen for Down syndrome in pregnancy.

00:10 We're going to discuss those methods and talk about why we would choose one over the other.

00:14 So the first is the free fetal DNA test.

00:18 Basically, we have found that there is free fetal DNA circulating in mom's blood.

00:23 So mom's blood is drawn, usually after 10 weeks of gestation and you're able to see the actual carrier type of the fetus.

00:30 So you know with a 99% accuracy, if the fetus has Down syndrome.

00:35 You're also able to find out the sex of the baby at that time.

00:39 So the next option for screening is integrated, sequential, or contingency screening.

00:44 These are all grouped the same way.

00:46 But they are performed just a little bit differently.

00:48 And it can be confusing.

00:49 So let's go over that.

00:52 So this test is performed between 11 weeks and 13 weeks and 6 days.

00:56 Has a detection rate of 96%.

00:58 Now the test involves performing a test called the Nuchal Translucency.

01:02 That's an ultrasound that measures the skin fall behind the baby's neck.

01:06 That is done along with blood work, PAPPA and beta-hcg.

01:11 Now the difference in the test.

01:13 So the integrated screening takes the result of that 1st trimester test.

01:17 Again the nuchal translucency and those two hormones PAPPA and beta-hcg.

01:22 And looks at the blood work at the 2nd trimester which is essentially the quad screen, integrates them together, to give a patient's risk of having a fetus with Down syndrome.

01:32 Now this is done as probability so there is no value that's high or low, correct or incorrect.

01:37 Because this is done as probability.

01:40 Sequential screening means that you take the results of the 1st trimester, you get a result.

01:45 You take the results of the 2nd trimester and get a result.

01:50 And overall you're given a result.

01:53 That's a little bit different than the integrated.

01:55 Where with integrated you don't get a result until the 2nd trimester.

01:59 The contingency screen is done in the 1st trimester.

02:03 And if the risk of having Down syndrome is low, the 2nd trimester test is not done.

02:08 Only an alpha-fetoprotein is done in 2nd trimester to determine if there is a risk of nuchal translucency.

02:14 Together these each have a detection rate of about 95 to 96%.

02:19 The 3rd test is the Quad screen.

02:22 This is performed between 15 weeks and 22 weeks and 6 days of gestation.

02:27 Mom's blood is drawn, looking for four different hormone levels.

02:31 That hcg, estriol, inhibin A and alpha fetal protein.

02:35 This has a lowest detection rate, it's 81%.

02:39 And again this is done as probability.

02:41 So it looks at a mother of that ethnicity, at that gestational age with those four hormone levels to tell the probability of the baby having Down syndrome.

02:51 So you can see amongst the three tests, the free fetal DNA is the most accurate.

02:57 However, because it is a little bit newer, it is the most expensive test of the three.

03:02 We do recommend this test for our women that are over 35 years old as they have an increase risk for Down syndrome.

03:09 The Quad screen can be used if a patient enters pregnancy after 13 weeks and 6 days and is not able to get the integrated sequential or contingency screen.

03:23 Now these previous discussed tests are meant for screening purposes only.

03:29 So if they are abnormal then invasive testing is recommended.

03:33 So what are options for invasive testing.

03:36 Well, there's two.

03:37 Chorionic villi sampling and Amniocentesis.

03:40 So Chorionic villi sampling is performed between 10 to 14 weeks of gestational age.

03:45 And as you can see here in the picture, a needle is inserted into mom's belly and there's blood that's drawn from the chorionic villi.

03:51 Karyotyping is performed to see if the fetus indeed has Down syndrome.

03:56 The risk of this procedure is pregnancy loss.

03:59 But we are able to do an early gestation.

04:02 The next invasive test is amniocentesis.

04:06 Now with this in a similar manner, as you can see here in the picture, a needle is placed inside mom's belly.

04:13 Amniotic fluid is drawn.

04:15 And again the carrier type is performed.

04:17 This test is usually down around 15 weeks gestational age.

04:20 And again it is confirmation to determine the patient has a fetus with Down syndrome.

04:25 The risk to go along with this procedure are bleeding, infection, placental abruption and again fetal loss.

04:31 It was previously thought that with chorion villi sampling that there was an increased risk of fetal loss compared to amniocentesis.

04:38 But now we know that the risk is about the same.

04:42 So let's go through a study here.

04:45 A 24 year old gravida 1 para 0 female at 8 weeks and 3 days of gestation, presents for an antenatal visit.

04:53 She has a sister with Down syndrome.

04:55 And is concerned that the baby may be at increased risk.

04:58 The best non-invasive screening option for this patient is: A. Free fetal DNA.

05:05 B. Integrated screening.

05:07 C. Quad screen.

05:09 or D. Amniocentesis.

05:11 Well, let's think about this options.

05:13 The free fetal DNA, we've already said, has a 99% detection rate for Down syndrome.

05:19 Integrated screening has about a 95%.

05:21 And then Quad screen has about an 81%.

05:25 Amniocentesis is the most accurate, because it is looking at the carrier type and it is the test for diagnosing Down syndrome.

05:32 But we have to go back to the stem of our question.

05:34 Amniocentesis is not a non-invasive test.

05:38 So based on our options, the answers is A.

05:41 The Free fetal DNA test.