- Why would my doctor send me to the Perinatal Center?
- What is first trimester screening?
- What are chromosomal problems?
- What is chorionic villus sampling?
- What is amniocentesis?
- What is fetal monitoring and when is it used?
- How often should an ultrasound be done in pregnancy?
Why would my doctor send me to the Perinatal Center?
Most patients who come to the Perinatal Center do not have complicated pregnancies, but are seeking the highest quality ultrasound and ultrasound interpretation. Patients may also be referred for first trimester screening, genetic counseling about maternal age or other family concerns, a previous high risk pregnancy, or a maternal health problem which may affect a pregnancy. Most patients will come to the Perinatal Center once or twice during their pregnancy, and then follow-up with their own obstetrician. Some patients, however, may need more frequent ultrasounds and advice.
First trimester screening is a new approach to identify pregnancies at higher risk for having a baby with a chromosomal problem. Patients are referred at 11-14 weeks of pregnancy for an ultrasound, which measures a fold of skin at the back of the baby's neck. This measurement, in association with the mother's age-related risk for a chromosomal problem, is combined with the measurements of proteins tested in the mother's blood (by fingerstick), to give an adjusted risk for having a baby with a problem such as Down syndrome.
This test is similar to the quad screen (done at 15-20 weeks of pregnancy), but provides information earlier. The first trimester screen can identify up to 90 percent of Down syndrome pregnancies. Results are available in two to four days.
Chromosomes are structures which organize our genetic information (genes). There are 46 in every cell, arranged into 23 pairs, so that each chromosome exists in two copies-one from the mother and one from the father. Each chromosome contains between 300-5,000 genes, and the number and the shape of each chromosome is very important. Missing a chromosome-even a piece of a chromosome-or having an extra chromosome can result in a very serious problem. Many pregnancies which have extra or missing chromosomes miscarry, but some may survive through the pregnancy, particularly those involving an extra chromosome 21 (Down syndrome) or extra chromosomes 13 or 18 (Trisomy 13 or Trisomy 18). As mothers get older, they are at greater risk for having pregnancies with missing or extra chromosomes, but the problem can occur at any age.
Chorionic villus sampling (CVS) is a way of identifying a baby's chromosome number, or whether it carries a genetic disorder identified in a family. It is done early in pregnancy, between 10-13 weeks, and involves taking a small sample of the placenta of the baby. The placenta and baby come from the same original cells, and have the same chromosomes. Depending on the location of the placenta, the sample is taken by a tiny plastic tube passed through the mother's cervix (lower part of the womb) from the vagina, or by a thin needle passed through the wall of the mother's abdomen into the womb. Results come back in about seven days. The test is very straightforward, but carries a small risk, between 1/100-1/200 risk for fetal loss, because of the very early nature of the pregnancy. Mothers who are Rh negative will need a Rhogam injection after the procedure.
Amniocentesis is another way of identifying a baby's chromosome number, or if it carries a genetic disorder. The test is performed slightly later in pregnancy, after 15 weeks. It involves using a very thin needle to pass through the wall of the mother's abdomen into the amniotic sac (water bag) around the baby. The fluid or water around the baby contains the baby's urine, which it passes naturally into the sac. The urine contains cells which come from the baby's skin or bladder. By taking a sample of the fluid, these cells of the baby can be obtained and sent for chromosomal study. The results come back in two to four days from the preliminary testing (Fluorescent In-situ Hybridization or FISH) for the most common chromosomes which can be duplicated or missing: 13, 18, 21, X and Y. The final results, looking at all the chromosomes, return seven to 10 days later. Amniocentesis has a very low risk of fetal loss, about 3 chances in 1,000. Mothers who are Rh negative will need a Rhogam injection after the procedure.
Fetal monitoring is used to evaluate if a fetus is showing stress in a pregnancy. It has two parts: a monitored part or "non-stress test (NST)," and an ultrasound part or "biophysical profile (BPP)." Some high-risk pregnancies, such as those with fetal abnormalities, maternal hypertension, diabetes, other serious maternal disease, or those which have gone past the due date, have an increased risk for developing problems with the placenta (the structure which connects the fetus to the mother and enables nourishment to pass from the mother's blood to the fetus).
A non-stress test can detect if the placenta is showing weakness, by measuring how the fetal heart rate changes when the fetus moves. The mother is placed in a comfortable chair, and two monitors are placed on her abdomen, over the area of the fetal heart and near the top of the uterus. These are held in place with soft straps. The fetal heart monitor measures the heart beat continuously, and shows a typical increase in heart rate when the fetus moves. The uterine monitor measures if the uterus is contracting. If the fetal heart rate does not increase with movement or if it drops with a contraction, this can be a sign of fetal distress.
A biophysical profile uses an ultrasound to show if the fetus' limbs and body are moving, how much fluid is around the fetus and if the fetus shows breathing movements. Each observation is given a score. If the score is 6-8 out of a possible 8, then the fetus has a very low chance of distress in the next 72 hours. If the score is 4 or less out of 8, then the test needs to be repeated, or the fetus should have more extensive observation-or even delivery.
There are no strict rules which require a mother to have an ultrasound in pregnancy. Some insurance companies will only pay for an ultrasound if there is a problem in the pregnancy, such as uncertain dates, bleeding, fetal abnormalities, abnormal screening tests, poor or excessive fetal growth or maternal health problems. If first trimester screening is desired, that is done at 11-14 weeks of pregnancy.
The best time to look at the fetus to study its anatomy is around 18-20 weeks. (If an ultrasound is not obtained until the last trimester, sometimes the fetus is so large that clear details of the anatomy are hard to see). Some maternal health conditions, such as chronic hypertension, systemic lupus erythematosus (SLE) or diabetes may affect fetal growth; these pregnancies will need serial ultrasounds for fetal growth every 4 weeks. In addition, if a fetus is found to have an abnormality, or has decreased or increased amniotic fluid or growth, serial ultrasounds every 3-4 weeks are recommended to evaluate growth as well.