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 risk 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 the Genomics Division of the Health Department of Western Australia if you are uncertain.

Other benefits of First Trimester Screening include:

1. Accurate confirmation or establishment of pregnancy dating
2. Diagnosis of multiple pregnancy and placentation with stratification of multiple pregnancy risk
3. Early review of the fetal structures
4. Assessment of your pelvic organs (eg to check for fibroids or ovarian cysts)

What are chromosomal disorders?

Normal Male Karyotype


Normal Female Karyotype

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 47chromsomes 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.

• 
  Trisomy 21 Karyotype
  in a male
• 
  Turner Syndrome
  (Monosomy X) Karotype
• 
  Trisomy 18 Karoytpe
  in a Male
• 
  Trisomy 13 Karyotype
  in a Male

What does the First Trimester Screening test involve?

The combined First Trimester Screening test has two separate components:

1. A blood test (ideally taken at 10 weeks gestation)
2. 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 risk 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 risk (the most likely result) or increased risk (this occurs in 3-5% of pregnancies). For women whose pregnancies screen at increased risk a specific test (chorion villus sampling  or amniocentesis is required to actually determine if the fetus is affected. In <5% of women whose pregnancies screen at increased risk 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

1. Free βhCG (a part of the pregnancy test hormone)
2. 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 is a problem with the baby (eg. Down Syndrome or a structural heart defect).

• 
  12 week embryo for crown-rump length
• 
  Normal Nuchal Translucency

In addition, the ultrasound will review the fetus to:

1. Ensure the fetal heart is beating
2. Check the dating of the pregnancy
3. Count the number of fetuses in the uterus (looking for twins or more!)
4. Assessment of the early fetal structures ( this is best done at 12-13 weeks)
5. 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?

At present, combined First Trimester Screening is the most sensitive and specific screening test for fetal Down Syndrome. This test should 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 risk proceed to chorionic villus sampling or amniocentesis. 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% 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-risk 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 risk 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 risk First Trimester Screening test does not mean your baby will be born healthy (although most babies are very healthy at birth).