What common technique can be used to confirm the presence of an embryo or fetus within the womb?

Prenatal diagnostic testing involves testing the fetus before birth (prenatally) to determine whether the fetus has certain abnormalities, including certain hereditary or spontaneous genetic disorders.

• Measurement of certain substances in the pregnant woman’s blood plus ultrasonography can help estimate the risk of genetic abnormalities in the fetus.

• These blood tests and ultrasonography may be done as part of routine care during pregnancy.

• If results of these tests suggest an increased risk, doctors may do tests to analyze the genetic material of the fetus, such as amniocentesis and chorionic villus sampling.

• These genetic tests are invasive and have certain risks for the fetus.

Some prenatal diagnostic tests, such as ultrasonography and certain blood tests, are often part of routine prenatal care. Ultrasonography and blood tests are safe and sometimes help determine whether more invasive prenatal genetic tests (chorionic villus sampling Chorionic Villus Sampling Prenatal diagnostic testing involves testing the fetus before birth (prenatally) to determine whether the fetus has certain abnormalities, including certain hereditary or spontaneous genetic... read more , amniocentesis Amniocentesis Prenatal diagnostic testing involves testing the fetus before birth (prenatally) to determine whether the fetus has certain abnormalities, including certain hereditary or spontaneous genetic... read more , and percutaneous umbilical blood sampling Percutaneous Umbilical Blood Sampling Prenatal diagnostic testing involves testing the fetus before birth (prenatally) to determine whether the fetus has certain abnormalities, including certain hereditary or spontaneous genetic... read more ) are needed. Usually, these more invasive tests are done when couples have an increased risk of having a baby with a genetic abnormality (such as a neural tube defect Neural Tube Defects and Spina Bifida Neural tube defects are a certain type of birth defect of the brain, spine, and/or spinal cord. Neural tube defects can result in nerve damage, learning disabilities, paralysis, and death. The... read more ) or a chromosomal abnormality (particularly when the woman is 35 or older). However, many doctors offer this type of testing to all pregnant women, and any pregnant woman can request it. These tests have risks, although very small, particularly for the fetus.

Couples should discuss the risks with their health care practitioner and weigh the risks against their need to know. For example, they should think about whether not knowing the results of testing would cause anxiety and whether knowing that an abnormality was not found would be reassuring. They should think about whether they would pursue an abortion if an abnormality was found. If they would not, they should consider whether they still want to know of an abnormality before birth (for example, to prepare psychologically) or whether knowing would only cause distress. For some couples, the risks outweigh the benefits of knowing whether their baby has a chromosomal abnormality, so they choose not to be tested.

Markers are usually measured at 10 to 13 weeks of pregnancy (1st-trimester screening). Other markers are measured at 16 to 18 weeks of pregnancy (2nd-trimester screening).

Screening during the first trimester usually consists of

• Blood tests to measure levels of pregnancy-associated placental protein A (produced by the placenta) and beta-human chorionic gonadotropin in the pregnant woman’s blood

• Ultrasonography to measure a fluid-filled space near the back of the fetus’s neck (called fetal nuchal translucency)

Ultrasonography can help estimate the risk of Down syndrome and certain other chromosomal abnormalities. It can show whether the space at the back of the fetus's neck is enlarged. If it is, risk of these abnormalities is increased.

Alternatively, a blood test (called cell-free fetal nucleic acid [cfDNA] testing) may be done. For this test, small fragments of the fetus's DNA, which are present in the pregnant woman's blood in tiny amounts, are analyzed. This test can accurately determine the risk of Down syndrome and some other chromosomal abnormalities in couples with a high risk of having a fetus with a chromosomal abnormality. The test can be done as early as 10 weeks of pregnancy but can also be done later. Many doctors offer this test to all pregnant women.

During the 2nd trimester, markers in the pregnant woman’s blood are measured and sometimes ultrasonography is done to evaluate the risk that the fetus will have certain abnormalities.

Important markers include the following:

• Alpha-fetoprotein: A protein produced by the fetus

• Estriol: A hormone formed from substances produced by the fetus

• Human chorionic gonadotropin: A hormone produced by the placenta

• Inhibin A: A hormone produced by the placenta

The alpha-fetoprotein level in blood is usually measured in all women, even those who have had 1st-trimester screening or chorionic villus sampling. A high level may indicate an increased risk of having any of the following:

• A baby with a birth defect of the abdominal wall

• Confirming the length of the pregnancy

• Determining whether more than one fetus is present

• Determining whether the fetus has died

• Detecting many birth defects

Targeted ultrasonography using high-resolution equipment can be done at some specialized centers. It provides more detail and may be more accurate than standard ultrasonography, particularly for small birth defects. Targeted ultrasonography done during the 2nd trimester can help estimate the risk of a chromosomal abnormality. Targeted ultrasonography aims to identify certain structural birth defects that indicate an increased risk of a chromosomal abnormality. This test can also detect certain variations in organs that do not affect function but may indicate an increased risk of a chromosomal abnormality. However, normal results do not necessarily mean that there is no risk of a chromosomal abnormality.

If ultrasonography results are normal, a fetal problem is less likely, but certain conditions, such as neural tube defects, are still possible. Thus, whether ultrasonography results are normal or not, many doctors offer amniocentesis to all women.

A high alpha-fetoprotein level or the presence of acetylcholinesterase in the amniotic fluid suggests

• A neural tube defect

• An abnormality in another structure, such as the esophagus, kidneys, or abdominal wall

A high alpha-fetoprotein level plus acetylcholinesterase in the amniotic fluid indicates a high risk of

Sometimes the amniotic fluid sample is contaminated with blood from the fetus. This blood may increase the alpha-fetoprotein level even when the fetus does not have an abnormality, making the results hard to interpret. In such cases, the fetus may not have any abnormalities.

For the most accurate results, both groups of tests—1st-trimester tests and 2nd-trimester tests—are done, and results from both are analyzed together. Results are not given to the couple until 2nd-trimester results are available. However, if couples want information sooner, they can request a type of screening that provides results during the 1st trimester. Then screening in the 2nd trimester depends on the level of risk indicated by the results of 1st-trimester screening:

• Intermediate risk: Second-trimester screening is offered.

• Low risk: Second-trimester screening for Down syndrome is not offered because the 1st-trimester risk is so low.

Couples should remember that screening tests are not always accurate. They may miss abnormalities, or they may indicate abnormalities when none are present.

Several procedures can be used to detect genetic and chromosomal abnormalities. All, except ultrasonography, are invasive (that is, they require insertion of an instrument into the body) and have a slight risk for the fetus.

• Confirm the length of the pregnancy

• Locate the placenta

• Indicate whether the fetus is alive

• Determine how many fetuses are present

• After the third month, detect certain obvious structural birth defects, including those of the brain, spinal cord, heart, kidneys, stomach, abdominal wall, and bones

• In the 2nd trimester, detect structural defects that tend to indicate an increased risk of a chromosomal abnormality in the fetus (called targeted ultrasonography)

Ultrasonography is done before chorionic villus sampling and amniocentesis to confirm the length of the pregnancy so that these procedures can be done at the appropriate time during the pregnancy. During these procedures, ultrasonography is used to monitor the fetus and to guide placement of instruments.

At some specialized medical centers, targeted ultrasonography using high-resolution equipment can be done. For this test, experts carefully assess the fetus to check for structural defects that indicate an increased risk of a chromosomal abnormality. Targeted ultrasonography can provide greater detail than conventional ultrasonography. Thus, this test may detect smaller abnormalities, and abnormalities can be seen earlier, more accurately, or both.

One of the most common procedures for detecting abnormalities before birth is amniocentesis. It is often offered to women over 35 because they have a higher risk of having a fetus with chromosomal abnormalities than younger women. However, many doctors offer this test to all pregnant women, and any pregnant woman can request it, even if her risk is not higher than normal.

In this procedure, a sample of the fluid that surrounds the fetus (amniotic fluid) is removed and analyzed. Amniocentesis is usually done at 15 weeks of pregnancy or later. The fluid contains cells that have been shed by the fetus. These cells are grown in a laboratory so that the chromosomes in them can be analyzed. Amniocentesis enables doctors to measure the level of alpha-fetoprotein (a protein produced by the fetus) in the amniotic fluid. This measurement more reliably indicates whether the fetus has a brain or spinal cord defect than does measurement of this level in the woman’s blood.

Detecting Abnormalities Before Birth

Chorionic villus sampling and amniocentesis are used to detect abnormalities in a fetus. During both procedures, ultrasonography is used for guidance.

In chorionic villus sampling, a sample of chorionic villi (part of the placenta) is removed by one of two methods. In the transcervical method, a doctor inserts a thin, flexible tube (catheter) through the vagina and cervix into the placenta. In the transabdominal method, a doctor inserts a needle through the abdominal wall into the placenta. In both methods, a sample of the placenta is suctioned out with a syringe and analyzed.

In amniocentesis, a doctor inserts a needle through the abdominal wall into the amniotic fluid. A sample of fluid is withdrawn for analysis.

Before the procedure, ultrasonography is done to evaluate the heart of the fetus, to confirm the length of the pregnancy, to locate the placenta and amniotic fluid, and to determine how many fetuses are present.

A doctor inserts a needle through the abdominal wall into the amniotic fluid. Sometimes a local anesthetic is first used to numb the site. During the procedure, ultrasonography is done so that the fetus can be monitored and the needle can be guided into place. Fluid is withdrawn, and the needle is removed.

Occasionally, the amniotic fluid contains blood from the fetus. Such blood may increase the alpha-fetoprotein level, making the results hard to interpret.

Amniocentesis rarely causes any problems for the woman or the fetus. The following may occur:

• Soreness: Some women feel slightly sore for an hour or two afterward.

• Spotting of blood or leakage of amniotic fluid from the vagina: About 1 to 2% of the women have these problems, but the problems do not last long and usually stop without treatment.

• Needle injuries to the fetus: These injuries are very rare.

Amniocentesis can usually be done when a woman is pregnant with twins or even more fetuses.

In chorionic villus sampling, a doctor removes a small sample of the chorionic villi, which are tiny projections that make up part of the placenta. This procedure is used to diagnose some disorders in the fetus, usually between 10 and 12 weeks of pregnancy.

The main advantage of chorionic villus sampling is that its results are available much earlier in the pregnancy than those of amniocentesis. Thus, if no abnormality is detected, the couple’s anxiety can be relieved earlier. If an abnormality is detected earlier and if the couple decides to terminate the pregnancy, simpler, safer methods can be used. Also, early detection of an abnormality may give the couple more time to prepare for the birth of a child with special medical needs.

Before chorionic villus sampling, ultrasonography is done to determine whether the fetus is alive, to confirm the length of the pregnancy, to check for obvious abnormalities, and to locate the placenta.

A sample of the chorionic villi can be removed through the cervix (transcervically) or the abdominal wall (transabdominally).

• Through the cervix: The woman lies on her back with her hips and knees bent, usually supported by heel or knee stirrups, as for a pelvic examination. The doctor inserts a thin, flexible tube (catheter) through the vagina and cervix into the placenta. For most women, the procedure feels very similar to a Papanicolaou (Pap) test, but a few women find it more uncomfortable. This method cannot be used in women who have an active genital infection (such as genital herpes or gonorrhea).

• Through the abdominal wall: The doctor anesthetizes an area of skin over the abdomen and inserts a needle through the abdominal wall into the placenta. Most women do not find this procedure painful. But for some women, the area over the abdomen feels slightly sore for an hour or two afterward.

For both procedures, doctors use ultrasonography to guide them as they insert the catheter or needle and suction out the tissue sample with a syringe. The sample is then sent for analysis. Many women have light spotting for a day or two after either of these procedures.

Rarely, the genetic diagnosis is unclear after chorionic villus sampling, and amniocentesis may be necessary. In general, the accuracy of the two procedures is comparable.

Whether routine use of preimplantation genetic testing to screen embryos for chromosomal abnormalities increases the chance of successful pregnancy remains controversial.

The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

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