First Trimester Screening
Although the vast majority of pregnant women have a healthy baby, on occasions this does not occur. First Trimester Screening is an ultrasound and biochemical based test principally designed to provide an individual likelihood assessment of your pregnancy for fetal chromosomal problems. Not all women will elect to have this screening test and we suggest you discuss this examination with your doctor and review the written information provided by WA Health if you are uncertain.
Other benefits of First Trimester Screening include:
- Accurate confirmation or establishment of pregnancy dating
- Diagnosis of multiple pregnancy and placentation with stratification of multiple pregnancy risk
- Early review of the fetal structures
- Assessment of your pelvic organs (eg to check for fibroids or ovarian cysts)
What are chromosomal disorders?
Chromosomes contain the DNA which determines our genetic makeup. In humans each cell in our body contains 46 chromosomes, made up of 23 individual pairs, one of each pair derived from the mother and father. Our chromosomal makeup is determined at the time of conception and cannot be altered.
Occasionally a baby may receive too many or too few chromosomes at conception (this is termed medically “aneuploidy”), or portions of the chromosomes may be rearranged (this is known medically as “translocation”). Such events usually cause significant problems in the fetus, leading to spontaneous miscarriage or birth defects (for example abnormalities of the structure of the heart or brain).
The most frequent chromosome abnormalities seen include conditions such as Down Syndrome (Trisomy 21) in which the fetus has 47 chromsomes in each cell in its body due to an extra number 21 chromosome; Turner Syndrome (Monosomy X) in which the fetus has only 45 chromosomes in each cell in its body, the missing chromosome being one of the sex chromosomes; Edward Syndrome (Trisomy 18) in which the fetus has an extra number 18 chromosome in each cell in the body and Patau Syndrome (Trisomy 13) in which the fetus has an extra number 13 chromosome in each cell in the body). For more information, review the Genetic Services of WA website. Down Syndrome is the most common genetic cause of intellectual handicap in humans. The prevalence of these chromosome disorders (apart from Turner Syndrome) tends to increase as maternal age increases.
What does the First Trimester Screening test involve?
The combined First Trimester Screening test has two separate components:
- A blood test (ideally taken at 10 weeks gestation)
- An ultrasound examination (ideally performed at 12-13 weeks gestation)
By combining maternal age, the results of the blood test and the ultrasound each woman can be provided with a likelihood of the pregnancy being affected by a chromosomal disorder such as Down Syndrome. The test does not tell you that your pregnancy does or does not have this condition, it merely provides information that the pregnancy is at reduced probability (the most likely result) or increased probability (this occurs in 3-5% of pregnancies). For women whose pregnancies screen at increased probability a specific test (chorion villus sampling or amniocentesis) is required to actually determine if the fetus is affected. In only 5-6% of women whose pregnancies screen at increased probability for a chromosome problem will the fetus actually have an abnormal chromosome result.
What can we tell from the blood test?
The blood test, ideally taken at 10 weeks gestation (although it can be taken at any time between 9 and 13 weeks) is sent to the laboratory to measure the level of two fetoplacental hormones in the woman’s blood. These hormones are
- Free ßhCG (a part of the pregnancy test hormone)
- PAPP-A (Pregnancy Associated Plasma Protein A – a growth promoting hormone in the placenta).
The levels of these two hormones are frequently abnormal when the fetus has a serious chromosome problem.
What can we tell from the ultrasound?
The ultrasound is used to assess the size of the fetus, by measuring its length from head to bottom (Crown Rump Length). In addition, the nuchal translucency, which is a small echo free space at the back of the fetal neck, is measured. All fetuses have a nuchal translucency, but the larger the nuchal translucency measurement the higher the chance that there may be a problem with the baby (eg. Down Syndrome or a structural heart defect).
- Ensure the fetal heart is beating
- Check the dating of the pregnancy by measuring other fetal parts (eg. Biparietal Diameter of the head)
- Count the number of fetuses in the uterus (looking for twins or more!)
- Assessment of the early fetal structures (this is best done at 13 weeks or more)
- Review of the mother’s pelvic organs
When will I get the results of this test?
If you have had your bloods for the biochemistry component prior to the ultrasound we will be able to provide your results immediately after the completion of the ultrasound component. If you have not had your bloods taken then we will not be able to provide you with a result until we receive this information. It is therefore ideal that you have your blood test taken prior to the ultrasound (ideally at 10 weeks gestation).
How accurate is this test?
Combined First Trimester Screening may be offered to all pregnant women, regardless of age. It has the capacity to detect 85-95% of fetuses affected with Down Syndrome if all women who screen at increased probability proceed to chorionic villus sampling or amniocentesis (the detection rate is lower in multiple pregnancies). This is a superior test to screening for Down Syndrome purely on the basis of increased maternal age (ie women over the age of 35-37 years), in which only 30-50% of affected fetuses can be identified. Using maternal age alone, one fetus with a chromosomal disorder is diagnosed for every 100-150 amniocenteses performed. With First Trimester Screening, one fetus with a chromosome disorder is detected for every 13-20 invasive prenatal procedures.
Combined First Trimester Screening is a voluntary test for pregnant woman and not all women will feel this is an appropriate test for them. You should discuss this test with your partner and your doctor prior to deciding to proceed with the screening.
A low-probability First Trimester Screening result does not guarantee your baby does not have a chromosome problem. It is a screening test and stratifies your pregnancy into increased or decreased likelihood but is not a diagnostic test (ie. it does not tell that your baby does or does not have a chromosome problem).
Additionally, a low probability First Trimester Screening test does not mean your baby will be born healthy (although most babies are very healthy at birth).
Are there any new chromosomal testing modalities available?
The ability to assess the fetus for the common chromosomal problems (eg. Trisomy 21, Trisomy 18, Trisomy 13, Turner's syndrome, Klinefelter's syndrome) by assessing the amount of cell-free fetal DNA in the mother's blood (Non-Invasive Prenatal Testing or NIPT) is now available. This new form of testing appears to be highly sensitive for Trisomy 21 (detection rates >99%) and may be performed at virtually any time in the pregnancy from 10 weeks onwards. Most of the testing has been performed in women at increased probability of chromosomal problems (eg an increased risk First Trimester Screen, fetal anatomical problems seen on ultrasound, older women who are pregnant) and it has not been well studied as a general population screening test yet. The advantage of NIPT is the avoidance of an amniocentesis or CVS if the pregnancy is at increased probability for chromosomal problems - an amniocentesis is only required if the maternal blood test indicates an increased amount of free fetal DNA for one of the chromosomes tested. It is important to recognise that NIPT is a high-level screening test and therefore associated with false positive results (ie. reporting there is a problem when in fact there is not) and false negative results (ie. saying all is well when it is not). The chance of a false positive result increases in women who have NIPT when their pregnancy is at very low risk of a chromosome problem (ie a risk >1:2,000).
NIPT is now performed in Australia and is able to be accessed by women who desire this form of chromosome screening using some local pathology providers. Womens Imaging Services can assist women in accessing NIPT after specific counselling. The medical staff at Womens Imaging Services have no contracts with any of the companies providing NIPT. The NIPT costs $450-850 (depending upon the particular service provider and the technique used) with no Medicare rebate. Once blood is drawn it will take 5-8 working days before a result is available. In 3% of women a result will not be available from the first blood draw (particularly if you are very overweight) and a redraw is required.
A current position statement on Non-Invasive Prenatal Testing is available from the International Society for Prenatal Diagnosis.