pruebas de detección durante el embarazo

What are pregnancy screening tests?

A pregnancy screening test (also known as an antenatal check), aims to detect a disease or condition in the early stages before it causes significant problems, and where treatment can be offered. The potential benefits of any screening tests offered should outweigh any possible risks from the test.

Most pregnant women have normal test results. A small number of tests are abnormal. In these cases further assessment and/or treatment can be offered which may prevent serious problems. A midwife or doctor should discuss each test with you before it is done.

Are pregnancy screening tests compulsory?

No. You need to agree for the tests to be done. You can refuse to have a test if you do not want it.

What happens during a pregnancy screening test?

General history (talking to your doctor or midwife)

Early on in your pregnancy your doctor or midwife will ask various questions about your general health, family history and social history, and will ask about any previous pregnancies which you have had. Strictly speaking, this is not a test. However, it can raise issues which may need special attention during the pregnancy, which may prevent problems later in the pregnancy.

For example, if you have diabetes, or if you had pre-eclampsia in a previous pregnancy, you will need closer monitoring than usual during the pregnancy. If you have social or family problems such as domestic violence then you may need special help.

Physical examinations

• Weight and height. Early on in the pregnancy your doctor or midwife will check your weight and height. This is to see if you are significantly underweight or overweight which may increase the risk of developing problems during pregnancy.

• Assessing the growth of the baby. This is done by a midwife or doctor during routine antenatal checks. They use a tape to measure from the top of the growing womb (uterus) to the bone at the bottom of your tummy (pubic bone). If need be, una ecografía can more accurately keep an eye on growth if it does not seem to be progressing normally.

• Presión arterial. This is measured regularly during routine antenatal checks. Together with a urine test to detect protein, this screens for a complication of pregnancy called pre-eclampsia. See the separate leaflet called Pre-eclampsia.

• Examination of the baby's position. At about 36 weeks of pregnancy a doctor or midwife will examine your tummy (abdomen) to feel the baby to detect if the baby is lying bottom-down (breech). If so, treatment to turn the baby to a head-down position may be considered.

If you are otherwise healthy, other physical examinations are not routinely done. For example, routine vaginal or breast examinations used to be done in the past, but are not necessary. (They may be done if you have symptoms which warrant examination.)

Análisis de orina

Urine is checked at antenatal checks using a simple dipstick test to detect:

• Proteína. Protein in the urine may indicate early pre-eclampsia.

• Sugar (glucose) as an initial check for diabetes.

• Germs (bacteria). During pregnancy you can have an infection of the urine without symptoms (asymptomatic bacteriuria). This can increase the risk of problems later in pregnancy, such as early childbirth. It can be treated with antibiotic medication.

Análisis de sangre

A sample of blood is taken early on in pregnancy and put into several tubes. These are sent to the laboratory to check for:

• Anemia. The common reason for anaemia is lack of iron, which can usually be treated easily with iron tablets.

• Blood group including rhesus D status and red cell antibodies. If you are rhesus D negative and your baby is rhesus positive then you may form anti-D antibodies in your bloodstream. These are not dangerous in the first pregnancy but can attack the blood cells of a baby who is rhesus D positive in any future pregnancy. To prevent this you will usually be offered anti-D injections later in your pregnancy.

• Rubella status. This test checks for antibodies to German measles (the rubella virus). If antibodies are present it means that you are immune to this infection. If you are not immune, when you are pregnant you should keep away from anyone who may have rubella and consider being immunised.

• Haemoglobin disorders. The blood test can detect if you're a carrier of anemia de células falciformes, talasemia or another haemoglobin disorder.

• Infecciones. Some uncommon but serious infections are checked for. These are:
  

  • el VIH. You can be infected with HIV for years before it causes symptoms. The risk of passing this virus on to your baby can be greatly reduced with treatment during pregnancy, and delivery by caesarean section.

  • Hepatitis B. Many people are carriers of the hepatitis B virus but have no symptoms. In some cases it can cause serious liver damage. If you have this virus, it may be prevented from affecting your baby if the baby is immunised at birth.

  • Sífilis. This is an uncommon sexually transmitted infection. Again, you can be infected with this germ (bacterium) without realising and pass it on to your baby. It can develop into a serious illness. It can be treated with antibiotics.

• Diabetes. If you have had a history of diabetes in a previous pregnancy (diabetes gestacional) then you will be tested for diabetes by a blood test in future pregnancies. In addition, you may be offered a test for diabetes if you have risk factors for diabetes; for example, if you:

  • Are muy por encima de su peso.

  • Have had a heavy baby in the past (≥4.5 kg).

  • Have a close family relative with diabetes.

  • Are from certain ethnic groups.

A repeat blood test at about 28 weeks is usually offered to check again for anaemia and red cell antibodies.

Ecografías

Una ecografía is a safe and painless test which uses sound waves to create images of structures inside your body, such as an unborn baby. Two scans are normally offered:

• An early pregnancy scan works out when your baby is due. The scan is usually done at around 10-13 weeks. This early scan confirms your baby's heart is beating and how many babies are growing inside your womb (uterus).

• A fetal anomaly scan is offered later in your pregnancy. The scan is usually done at around 20 weeks. This looks for any developmental problems with your baby. There are variations across the UK as to exactly when and how fetal anomaly scanning is done.

Placenta previa

Placenta praevia means that the placenta is covering the opening from the womb to the neck of the womb (cervix). This can cause serious problems during childbirth. If an earlier scan using sound waves (an ultrasound scan) indicates that you may have a placenta praevia, a repeat scan at 36 weeks of pregnancy may be advised. This is to clarify the position of your placenta before delivery.

Screening tests for the baby

The programme for checking for abnormalities in your baby differs across the UK. You can choose not to have some of the tests done if you prefer. The scan at around 20 weeks as described above checks for any problems with your baby's heart, kidneys, spine, etc and can check for conditions such as cleft lip/palate.

Combined Test

In England, Scotland and Wales, you will be offered a test to screen your baby for genetic abnormalities. Genetic means that the condition is passed on through special codes inside cells called genes.

The conditions which are checked for are:

• Down's syndrome, also called trisomy 21.

• Edwards' syndrome, also called trisomy 18.

• Patau's syndrome, also called trisomy 13.

The test is called the combined test because it combines a blood test and an ultrasound scan. The ultrasound scan measures the fluid at the back of the baby's neck to determining the "nuchal translucency". The combined test can be done between 10 and 14 weeks at the same time as the blood tests and scan mentioned above. Your age and the results of these two tests are used to work out the chance of your baby having Down's syndrome, Edwards' syndrome or Patau's syndrome.

If you miss this time window, these conditions can also be screened for with a blood test between 14 and 20 weeks of pregnancy. This is called the quadruple test and only screens for Down's syndrome. It is less accurate than the combined test. In Northern Ireland some centres offer these tests in the same way, or you may be able to request them within the same time frames.

Síndrome de Down

Síndrome de Down is a genetic chromosomal problem that some people are born with. A person with Down's syndrome can usually be recognised by their typical features. It can also cause learning disability and certain medical problems. Someone with Down's syndrome has an extra copy of chromosome number 21 in the cells of their body.

Edwards' and Patau's syndrome

Babies with Edwards' syndrome and Patau's syndrome have an extra copy of chromosome number 18 or 13 respectively. Unfortunately, babies with Edwards' or Patau's syndromes have severe abnormalities and do not usually survive very long.

Pregnancy screening test results

The combined test results will provide you with information on the risk of your unborn baby having Down's syndrome, Edwards' syndrome or Patau's syndrome. It does not give you a definite diagnosis or a definite answer. The test tells you if your baby has a higher chance or lower chance of having one of these conditions.

A lower chance result does not mean there is no chance of your baby having Down's syndrome, Edwards' syndrome or Patau's syndrome. It means the chance is less than 1 in 150.

If your combined test result shows that the chance of having a baby with Down's syndrome, Edwards' syndrome or Patau's syndrome is more than 1 in 150 this is called a higher chance result. This does not mean your baby definitely does have one of these syndromes. You will then be offered further tests which you can choose whether to have or not.

There is a second screening test called non-invasive prenatal testing (NIPT). This can give you a more accurate screening result which might help you to decide whether to go on to have a diagnostic test or not.

Only diagnostic tests will give you a definite answer (these are amniocentesis y muestreo de vellosidades coriónicas. These are not screening tests. They are, however, included here as they may be offered if a screening test is abnormal. Diagnostic tests are invasive prenatal tests and carry a small risk of miscarriage (approximately 1 in 200).

You do not have to have a screening test for Down's syndrome or other genetic conditions if you do not want one. If you have a screening test and it shows a higher chance of Down's syndrome, Edwards' syndrome or Patau's syndrome you do not have to a diagnostic test. Find out more about prenatal screening and diagnosis of Down's syndrome.

A final point

The above tests are the usual routine tests offered to all pregnant women. If you have symptoms or problems which suggest pregnancy complications, various other examinations and tests may be advised. See your midwife or doctor if you have any concerns about your pregnancy, or if you wish to discuss any tests in more detail.