- Medical services
- Definition of medical terms
First Trimester (as used on this website): the pregnancy period from the first day of your last menstrual period to 13 weeks and 6/7 days. Note that the fetus is fully formed by 12 weeks of pregnancy. From this point on, the fetal organs simply grow in size and mature in function.
Second Trimester (as used on this website): the pregnancy period from 14 weeks to 27 weeks and 6/7 days.
Ultrasound: a non-invasive evaluation of your pregnancy by the use of sound waves, also referred to as sonography, performed by abdominal and/or vaginal approaches. Sonography, is usually performed by two-dimensional (2-D) imaging. Three-dimensional (3-D) and four-dimensional (4-D, or live 3-D) imaging is occasionally helpful and neccessary in further evaluation of an abnormality noted on 2-Dimaging or in evaluating certain fetal structures that are not easily seen on 2-D imaging such as the fetal palate, the roof of the mouth .
First Trimester (genetic) Ultrasound: is a detailed ultrasound of the fetal anatomy that is performed preferably between 12 weeks and 13 weeks 6 days. The evaluation is performed both through abdominal and vaginal sonography. If the fetus appears normal on this ultrasound, then the likelihood of any chromosomal, genetic, or fetal structural abnormality is decreased.
Second Trimester (genetic) Ultrasound: is a detailed ultrasound of the fetal anatomy that is performed preferably between 14 and 20 weeks. The evaluation is performed usually through abdominal sonography, but at times vaginal sonography is required to complete the assessment. If the fetus appears normal on this ultrasound, then the likelihood of any chromosomal, genetic, or fetal structural abnormality is decreased.
Aneuploidy: refers to genetic conditions where there are an abnormal number of chromosomes in the nucleus of the cell. Down syndrome, Trisomy 18, and Turner syndrome are examples of aneuploidies. Our genes are contained in segments called chromosomes. Normally we have 46 chromosomes (23 pairs) in the nucleus of each cell of our body, except for sperm and ova (eggs) that have 23 chromosomes. The normal set of 46 chromosomes is paired into 23 pairs: twenty-two autosomal (non-sex chromosomes) pairs numbered from 1 to 22, and two sex chromosomes either XX (female) or XY (male). At the time of fertilization of the egg by a sperm, sometimes instead of ending up with the expected 46 chromosomes the fetus ends up with 45, 47, 69 or other number of chromosomes in each cell. Such a fetus is referred to as having aneuploidy. If the extra chromosome for example is that of number 21, then the fetus has three chromosomes of number 21, i.e. Trisomy 21, which is Down syndrome. If the extra chromosome is that of number 18, then the fetus has Trisomy 18.
Maternal age related risk for having a baby with Down syndrome: The large majority of babies with Down syndrome are born to families without prior history of that condition, in other words it is not an inherited condition. The risk of having a baby with Down syndrome is directly related to the maternal age, i.e. the age of the eggs at time of fertilization. The risk of having a baby with Down syndrome increases with increasing maternal age at time of conception. Fifty percent of babies with Down syndrome are born to women younger than the age of 35 years, and fifty percent are born to those 35 years and older. The reason for the high percentage of babies with Down syndrome in women less that 35 years of age, is the higher rate of child bearing in that group. Down syndrome is the most common chromosomal aneuploidy born to women of any age group. It is for that reason that we have concentrated our efforts in finding non-invasive screening tests to determine those pregnancies at high risk of having Down syndrome so that we may offer women conclusive chromosomal testing, i.e. chorionic villus sampling (CVS) or amniocentesis.
(Aneuploidy) Screen: is a non-invasive test that calculates your risk of having a baby with certain aneuploidies such as Down syndrome and Trisomy 18. The screening test can be a blood test, an ultrasound, or a combination of both. There is no risk of losing your pregnancy from a screening non-invasive test.
Nuchal Translucency (NT): is the area underneath the fetal skin in the neck that is measured by abdominal or vaginal ultrasound and requires nearly 5 minutes to image. If this area is larger than expected then the risk of aneuploidy, genetic syndromes and heart defects is increased. If the area is normal in size, then the risk of Down syndrome is decreased by 70-80% from the maternal age expected risk.For example, if a 35 year old women, has a fetus with a normal NT, then her age related Down syndrome risk drops from an average of 1/250.
PAPP-A & Free beta hCG: are two hormones that are produced by the placenta. There level in the maternal serum, between 11 weeks 3 days and 13 weeks 6 days of pregnancy, indicates if there is a high or low probability for the fetus of having Down syndrome or Trisomy 18. This test is performed by a finger stick and is associated with minimal discomfort.Recent studies have also shown a high correlation of poor pregnancy outcome with very low PAPP-A levels such as fetal death, poor fetal growth in the uterus, premature delivery, and aneuploidies other than Down syndrome and Trisomy 18.
First Trimester aneuploidy screen: is a test that combines the NT (see 7- above) and PAPP-A/Free beta hCG (see 8- above) information to calculate risks of Down syndrome and Trisomy 18. This screening is performed between 11 weeks 3 days and 13 weeks 6 days. It allows for the prenatal detection of 85-90 % of fetuses with Down syndrome, if all patients whose test shows high probability of Down syndrome agree to an invasive procedure, CVS or amniocentesis. High probability for Down syndrome on this test is when the risk exceeds 1 in 254, e.g. 1 in 200, 1 in 90, and so on. This screening test has a 5% false positive reading for Down syndrome, i.e. in 5 out of every 100 women who have this test, the results will incorrectly (falsely) indicate a high probability for Down syndrome. Therefore, 5 % of women undergoing the test will end up with unnecessary invasive testing to rule out Down syndrome.
AFP expanded program, or Triple marker screen (AFP, uE3, & hCG): is a California State maternal serum test offered to all pregnant women irrespective of their age, between 15 weeks and 20 weeks of pregnancy. The three hormones are produced by the placenta. The maternal serum level of these hormones, allows calculation of the probability that the fetus has Down syndrome, Trisomy 18, and neural tube defect (e.g. spina bifida). The probability of these conditions is reported as “positive” or “negative” based on arbitrary probability cut offs. A “positive” report for Down syndrome does not mean that the fetus has that condition. It simply means that the calculated probability is higher than 1 in 190, e.g. 1 in 100, that the fetus has Down syndrome. Similarly, a “negative” report does not mean that the fetus does not have Down syndrome. It simply means that the probability of the fetus having Down syndrome is less than 1 in 190, e.g. 1 in 1000. The risk for neural tube defect is not reported as a probability, but simply as “positive” or “negative”, though these results too only indicate increased or decreased risk, respectively, and do not indicate the presence or absence, respectively, of neural tube defects. The Triple marker screen will soon be replaced by the Quadruple marker screen which assesses, in addition to the aforementioned three hormones, the hormone Inhibin. The Triple marker screen has a high false positive Down syndrome screening rate in women over 34 years of age. The Quadruple screen on the other hand has as low a false positive Down syndrome screening rate as the First Trimester aneuploidy screen.
Second Trimester aneuploidy screen: Combines the information from the Triple, or the Quadruple, Marker Serum screen or the maternal age, and the second trimester genetic ultrasound. In the absence of certain fetal ultrasound findings, the risk for Down syndrome specifically, can be decreased by 60-80% from the Triple/Quadruple Serum Marker or maternal age calculated risk for Down syndrome. For example, if a patient had a “negative” report on the Triple/Quadruple Serum Marker for Down syndrome, with a probability of having a baby with Down syndrome calculated at 1 in 500, then the final risk for Down syndrome, after a normal ultrasound, becomes 1 in 1250 to 1 in 2500.
Chorionic Villus Sampling: is an invasive test where a biopsy of the placenta is taken either through the abdomen with a needle or through the cervix with a catheter. The test is associated with near 1% pregnancy loss.
Amniocentesis: is an invasive test where amniotic fluid is taken with a needle, inserted into the pregnancy sac (amniotic sac) through the abdomen. The test is associated with 1/100 to 1/400 pregnancy loss depending on when the test is performed. The risk is higher in early amniocentesis that is usually performed prior to 15 weeks of gestation.