The standard in the U.S.A has for years recommended an ultrasound of the pregnancy at around 18 to 20 weeks’ gestation for women considered at high-risk for genetic, obstetrical, or medical complications. However, in the advent of high-resolution ultrasound machines and in the hands of experienced sonographers, it is possible to detect many fetal abnormalities starting at 12 weeks of the pregnancy and on (refer to First Trimester Fetal Ultrasound section).
Whether a first trimester ultrasound was performed or not, a second trimester ultrasound is essential in ruling out abnormalities not noted or not present during the first trimester evaluation. For patients with low risk pregnancies and for patients with a normal early pregnancy ultrasound, the second best time to evaluate the fetus is around 20 weeks gestation. However, if the patient has a high -risk condition such as age over 34 years or diabetes, or if the patient has strong family history of genetic problems such as previous child with Down syndrome or spina bifida, then we recommend a near 16 weeks ultrasound and a 20-22 weeks ultrasound, in addition to the 12 week ultrasound if it was already obtained.
Ultrasounds performed in the second trimester allow for evaluation of normalcy of fetal anatomy and for evaluation of the presence of ultrasound markers of aneuploidy. Ultrasound markers of aneuploidy are ultrasound findings in the fetus that when present increase the risk of a chromosomal problem such as Down syndrome. Of their own they do not constitute fetal abnormalities and if the fetus has normal chromosomes then they are of no consequence to the baby after birth.
If a fetus appears to have a structural abnormality on ultrasound, the patient and her partner will be fully advised about that condition and will be given appropriate genetic counseling so that they may determine their desire for amniocentesis. If the fetus does not have any structural abnormality but shows ultrasound markers of aneuploidy, then depending on the marker the risk of Down syndrome and Trisomy 18/13 can be calculated. The current medical literature supports that pregnant women, of any age, should have either the Triple Marker Serum (Expanded AFP Program) or Quadruple Marker Screen ?, whether or not they had First Trimester Aneuploidy Screen ? . The Triple Marker Screen is obtained from three maternal serum hormone levels and entails a maternal blood draw. The Quadruple Marker Screen includes the hormone Inhibin in addition to the same three hormones obtained on Triple Marker Screen. Results require an average of one week. The Quadruple Marker Serum Screen reports separate risks for Down syndrome, Trisomy 18, and Neural Tube defect (e.g. spina bifida). The Triple Marker Screen reports in addition a risk for a rare condition SLOS. The combination of the Triple or Quadruple Marker Screen and second trimester ultrasound results allows for a composite calculation of risk for Down syndrome and Trisomy 18. It is also possible to obtain a more accurate composite risk for these syndromes from combining the first trimester aneuploidy screen with that of the second trimester.
It is important to understand the limitation of ultrasound in determining normalcy of the fetus whether structural, functional, or genetic. Even in the most experienced hands, at best 80-90% of abnormalities noted at birth are detected prior to delivery if all patients undergo three genetic ultrasounds at around 12, 20 and 30 weeks of pregnancy. Three consecutive “normal” genetic ultrasounds with normal maternal hormone levels and screens in the first and second trimester decreases the risk of having a baby with Down syndrome by at least 90-98% from the risks calculated by using maternal age. Certain serious conditions such as deafness, blindness, autism, mental retardation, among many others cannot be detected on ultrasound nor on invasive testing (CVS or amniocentesis). Other serious conditions such as Tay-Sachs, Fragile-X, Cystic fibrosis, among other genetic conditions are not routinely tested for on CVS or amniocentesis, but can be tested for if there is family history or if the parents specifically request these tests.
How to prepare for, and what to expect during, Second Trimester genetic Fetal Ultrasound in our office:
- Ultrasound preferably performed between 18 and 22 weeks
- Triple or Quadruple Marker Serum Screen performed best at 16 weeks, but anywhere between 15 and 20 weeks
- Ultrasound performed through abdominal and rarely vaginal approach
- Does not require a full bladder
- You may require 15-20 minutes to fill out paper work or for our staff to obtain insurance information in our office prior to your evaluation
- Your ultrasound requires an average of 30 minutes per fetus
- Mild discomfort during vaginal probe ultrasound, if required
- Allows for detection of many serious fetal structural abnormalities
- Allows for risk calculation of Down syndrome, Trisomy 18, Neural Tube defect, and SLOS when combined with maternal Triple Marker Serum Screen
- Allows for risk calculation of Down syndrome, Trisomy 18/13, and Neural Tube defect when combined with maternal Quadruple Marker Serum Screen
- Allows for risk calculation of Down syndrome, Trisomy 18/13, Neural Tube defect, and other syndromes when combined with first trimester aneuploidy screen and genetic ultrasound and second trimester maternal Marker Serum Screen
- Triple and Quadruple Marker Serum Screen requires maternal blood draw, usually performed at your Obstetrician’s or our office & results require average 7 days
- Amniocentesis offered based on many factors and our guidelines will be discussed during your consultation
The absence of apparent fetal abnormalities on ultrasound, a “negative” aneuploidy screen, and normal chromosomes do not delete all possible fetal abnormalities or genetic conditions, and do not guarantee normal organ functioning after birth but do decrease the risks for any such complications