The First Trimester Screen as Part of Prenatal Testing

The First Trimester Screen, also known as a combined first trimester risk assessment, is a non-invasive prenatal test to check the health of your baby. The screening uses a combination of an ultrasound, blood tests, and personal history to look for indications of fetal chromosomal abnormalities and cardiovascular defects.

First trimester screening is optional and is not considered one of the routine prenatal tests. Women who are considered high risk for babies with birth defects, or those who just want reassurance of a healthy pregnancy, may opt to have a First Trimester Screen.

First Trimester Screening: What does it show?
The First Trimester Screen is effective in screening for chromosomal abnormalities, such as Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13). It can detect signs of congenital heart defects that may or may not be associated with chromosomal abnormalities. It does not show the risk of neural tube defects, such as spina bifida.

A study published in the November 2005 issue of the New England Journal of Medicine indicated the First Trimester Screen detected between 82%-87% of chromosomal abnormalities, with a false positive rate of 5% or less, and was more accurate than second trimester screening.

It is important to note, the First Trimester Screen is a screening to look for risk factors for birth defects. It is not a diagnostic test. A First Trimester Screen that is considered positive, meaning there are enough risk factors present to indicate potential problems, does not necessarily mean your baby has a birth defect. It just means more testing is warranted. Results are given in the form of probabilities, such as a 1 out of 500 chance of chromosomal abnormality. That still means there is a 499 out of 500 chance the baby will be healthy.

First Trimester Screening: How is it done?
First trimester screening is done between pregnancy weeks 11 and 13, with the best results being achieved during week 11.

The first part of the First Trimester Screen is a blood test. The mother's blood is tested for hCg (human chorionic gonadotropin, a.k.a. 'the pregnancy hormone') and PAPP-A (pregnancy-associated plasma protein A). The levels of hCg decrease during normal pregnancies, but remain high or increase when chromosomal abnormalities are present. PAPP-A levels increase during normal pregnancies, but tend to be low or decrease in babies with chromosomal birth defects.

The second part of the First Trimester Screen is an ultrasound, which is used to measure the baby. The main measurements used are the 'crown-rump length' (the measurement from the top of the head to the bottom of the buttocks, which should be 45 to 84 millimeters) and 'nuchal translucency' (a measurement of the space between the soft tissue at the back of the neck and the skin). The chance of a chromosomal abnormality or heart defect increases when the nuchal translucency is wider than normal. Some doctors will also check the nasal bone of the fetus, looking for one that is too small or missing, but this is not as common.

The last part of the First Trimester Screen is to consider the risk factors of the mother. Her age and medical history, as well as the medical history of her family, are combined with the results of the blood tests and ultrasound to calculate the final probability score.

It takes about one week for the final screen results. The screening is considered positive when the probability is scored at 1 out of 280 or greater. If the result is positive, more invasive diagnostic tests, such as an amniocentesis or chorionic villus sampling, are commonly suggested.

The key to the First Trimester Screen is the combined results of tests and risk factors. One abnormal test on its own is not enough to determine the probability of birth defects.

First Trimester Screening: Why have it done?
The First Trimester Screen can assist in the early detection of birth defects. This allows the parents and doctor more time to decide on possible treatments for heart defects and to prepare for the care of a special needs child. In the case of a negative test with a low risk factor, it can give nervous parents some reassurance the pregnancy is progressing normally.

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Sources:
Mayo Clinic
American Pregnancy Association
American College of Obstetricians and Gynecologists

Healthline.com - Kenneth F. Trofatter, Jr., MD, PhD:
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