Invasive prenatal testing and prenatal care

• Invasive prenatal diagnosis means taking a sample of fetal or placental tissue or fluid so that a chromosomal or genetic diagnosis can be made: chorionic villus sampling (CVS) from the placenta and amniocentesis from the amniotic fluid.
• These tests are diagnostic (yes or no); screening tests only estimate chance. A positive screen must be confirmed by CVS or amniocentesis before any irreversible decision.
• CVS is usually done at 11 to 13+6 weeks; amniocentesis from 15 weeks. In experienced hands the added miscarriage risk for each is probably below 0.5%, rising to around 1% in a twin pregnancy. ^[1]
• Antenatal care around the tests matters as much as the tests: early booking by 10 weeks, regular checks (blood pressure, urine protein), screening for infections such as HIV, syphilis and hepatitis B, scans at 11 to 14 and 18 to 21 weeks, and enough time for informed consent.
• Do not do CVS before 10 weeks or amniocentesis before 15 weeks; early amniocentesis (before 15 weeks) caused more pregnancy losses and more talipes in randomised trials.
• Blood-borne virus results and viral load should be reviewed before any invasive test, because the test itself can transmit infection.

By the end of this page you should be able to:

1. State the indications, contraindications and gestational-age windows for CVS and amniocentesis.
2. Outline preparation, consent and the steps of each procedure in order.
3. Recognise the complications of invasive prenatal testing and how to manage them.
4. Plan the aftercare and follow-up after a sample has been taken.
5. Describe the routine antenatal visits, measurements and screening tests that surround these procedures.
6. Name the infections screened for in pregnancy and the guideline sources (RCOG, ACOG, NICE, WHO, CDC) that set these standards.

Prenatal diagnosis exists to give people and their clinicians information early enough to act on it: a woman with a raised-chance aneuploidy screening result, a fetus with a structural abnormality seen on ultrasound, or a known risk of an inherited genetic condition each has reason to be offered an invasive test.^[1] The goal is never a single "best" test. It is an informed choice that matches the person's wish for information with the test's safety, timing and limits. Every pregnant person can accept or decline any part of screening or diagnostic testing.

Prenatal genetic diagnostic testing is intended to determine, with as much certainty as possible, whether a specific genetic disorder is present in the fetus.^[2] It cannot identify every abnormality, and a single invasive test does not screen for everything. That is why the mid-pregnancy anomaly scan still matters even after a normal invasive result.

Screening estimates chance (it gives a risk such as 1 in 150); diagnostic testing gives a yes or no answer. Cell-free DNA screening is the most sensitive screening test for the common trisomies, but it remains a screen and a high-risk result is not a diagnosis. Always confirm with CVS or amniocentesis before making an irreversible decision.

• Chorionic villus sampling (CVS) takes a tiny sample of placental tissue (chorionic villi) under ultrasound guidance, usually by the transabdominal route, less often transcervically. It can be done in the first trimester, so an answer comes early.
• Amniocentesis withdraws amniotic fluid containing fetal cells. It is done from the second trimester and is the procedure of first choice from 15 weeks onwards in the evidence reviewed.^[6]

Both sample types can be sent for conventional karyotype and, when appropriate, microarray, with rapid preliminary results when clinically needed.

Offer amniocentesis or CVS for prenatal diagnosis for reasons that include:^[1]

• a higher-chance aneuploidy screening result (for example after combined first-trimester screening or a high-risk cell-free DNA result);
• a fetal structural anomaly seen on ultrasound that suggests an underlying genetic condition;
• a known risk of an inherited genetic disease;
• other indications judged case by case by the fetal medicine team.

The choice between CVS and amniocentesis depends mainly on gestational age and on individual counselling. Individualised counselling of the merits of CVS versus amniocentesis should be provided for women considering CVS in the later first-trimester window.^[1]

There are few absolute contraindications; the more important limits are gestational-age and safety rules:^[1][6]

• CVS should not be performed before 10+0 weeks, because of the small reported association with limb defects at very early gestation. ^[1]
• Amniocentesis should not be performed before 15+0 weeks, and early amniocentesis (before 15 weeks) should not be used as an early alternative because randomised trials found increased pregnancy losses and more congenital anomalies, especially talipes. ^[1]
• CVS is usually performed between 11+0 and 13+6 weeks. If required it can be performed between 14+0 and 14+6 weeks, with counselling about the merits of CVS versus amniocentesis in that window. ^[1]
• Persistent bleeding, active pelvic infection, an unplanned or unstable maternal condition, or a situation where the patient cannot give informed consent should be addressed first.
• With multifetal pregnancies, the risk–benefit balance and the technical difficulty are higher (see Special groups).

The clinician needs working knowledge of:

• the gravid uterus and the anterior abdominal wall;
• the placenta, its position and its relation to the fetus (anterior, posterior, fundal);
• the amniotic sac, the fluid space and the membranes, which remain separate as amnion and chorion until about 15 weeks; ^[6]
• the bladder, whose filling can improve the ultrasound view in the first trimester;
• the major abdominal and pelvic vessels and bowel loops to be avoided by the needle.

• high-resolution real-time ultrasound to guide every step;
• sterile skin preparation, sterile gloves and a disposable sterile needle or needle-cannula set;
• collection vessels (for amniotic fluid, or a sterile dish for villi);
• cell-culture media and transport to an accredited genetics laboratory that can provide conventional karyotype, and microarray where appropriate;
• cardiotocography or auscultation equipment for fetal heart rate before and after the procedure in later pregnancy.

Preparation and consent are as important as the needle tract. Under NICE and RCOG standards, women and their partners receive detailed counselling and pregnancy mapping by suitably trained professionals, and care is organised to national screening standards.^[1][3]

The following should be in place:

• Timing confirmed by ultrasound. Gestational age is verified; a non-viable pregnancy is excluded; multiple pregnancy and chorionicity are identified. The operator checks fetal heart rate and fetal position.
• Blood-borne virus review. When an invasive test is considered, screening results for blood-borne viruses, viral load and antigen tests should be reviewed, and the individualised risk of viral transmission should be discussed.^[1]
• Informed consent. The clinician explains the indication, what will happen, the benefits, the risks (including pregnancy loss, bleeding, infection and membrane rupture), the alternatives (including no testing), and the limits of the test (it cannot exclude every abnormality, and some results raise further questions). The right to decline without prejudice to care is stated clearly.
• Counselling style. Shared decision making with the woman (and her partner if she wishes), in her preferred language, with an interpreter when needed. Decisions that are contrary to the clinician's view are respected.^[3]
• Fasting and medications. No routine fasting is needed for amniocentesis; current long-term medicines are reviewed and any needed adjustment is discussed with the responsible clinician.
• Baseline fetal heart rate is recorded before and after the procedure in the second trimester.

The steps below are the common transabdominal route. Atraumatic, ultrasound-guided technique by a skilled operator is what keeps the risk low.^[1][6]

1. Ultrasound survey: confirm viability, single or multiple pregnancy, gestational age, and placental position.
2. Choose a needle path that avoids the fetus and reaches the placenta; prefer a route that does not traverse maternal bowel.
3. Clean the skin and, under asepsis, insert the needle into the placental mass under continuous ultrasound guidance.
4. Aspirate a small sample of villi; confirm on ultrasound that the sample looks adequate and that fetal heart rate is still present.
5. If the first pass fails, a second pass may be made; repeated passes raise the risk. Transcervical sampling may be chosen instead when it is technically easier.
6. Seal the sample, label it and send it promptly for culture and analysis.
7. Record fetal heart rate after the procedure and observe the woman before discharge.

CVS samples placental tissue; because the placenta can differ from the fetus (confined placental mosaicism, about 1 to 2 percent of cases), a mosaic or unexpected result should be confirmed by amniocentesis.^[5][6]

1. Ultrasound survey: confirm viability, gestational age, fetal position, number, amniotic fluid pocket and placental position.
2. Select a pocket of amniotic fluid free of fetal parts, cord loops and placenta.
3. Under continuous ultrasound guidance, insert a spinal-type needle through the abdominal wall into the amniotic cavity.
4. Withdraw a measured volume of amniotic fluid: enough for karyotype and, if needed, microarray; in practice 15 to 20 millilitres in the mid-trimester. Removing the needle and replacing the stylet before withdrawal reduces trauma.
5. Send the fluid promptly to the laboratory; keep it at room temperature, as cooling kills cells.
6. Assess fetal heart rate, and observe the woman before discharge.

Earlier in pregnancy the amniotic cavity is small: until 15 weeks there are two separate membranes, and only the inner (amniotic) sac should be aspirated because the outer sac does not contain enough living fetal cells. Whenever possible, amniocentesis is delayed to 15+0 weeks or later, when culture success was reported at 94 versus 82 percent before 15 weeks.^[6]

• Rest for the remainder of the day; normal light activity usually resumes the next day.
• Advise the woman to report promptly any bleeding, leaking of fluid, fever, strong or worsening pain, or reduced fetal movements.
• RhD-negative women who are not already sensitised receive anti-D prophylaxis after the procedure (in line with the local protocol derived from NICE anti-D guidance).^[3]
• Arrange clear routes for results: who will call, when, and how counselling will be offered, including non-directive counselling after a confirmed diagnosis.
• Document the procedure, the number of needle passes, the amount of sample, the operator and the laboratory.
• Keep routine antenatal care going: antenatal classes, immunisations, blood pressure and urine protein checks, fetal movement advice, and growth monitoring.

The complications below apply to both procedures; Cochrane data are given where available.^[6]

• Pregnancy loss. Second-trimester amniocentesis increased the risk of pregnancy loss in one trial (total pregnancy loss 3.2 versus 2.3 percent; spontaneous miscarriage 2.1 versus 1.3 percent), although the estimate is imprecise and came from a study over 30 years old.^[6] In experienced hands RCOG gives an added risk likely below 0.5 percent for either procedure and around 1 percent in a twin pregnancy.^[1]
• Early amniocentesis. Carried more total losses (7.6 versus 5.9 percent), more spontaneous miscarriages (3.6 versus 2.5 percent) and more anomalies, especially talipes (4.7 versus 2.7 percent), than second-trimester amniocentesis. It is not a safe substitute.^[6]
• CVS route. Transcervical CVS may carry a higher total pregnancy loss (14.5 versus 11.5 percent) than second-trimester amniocentesis and is more likely to fail technically (2.0 versus 1.1 percent sampling failure); transabdominal CVS showed no clear difference (6.3 versus 7.0 percent total losses).^[6]
• Bleeding or amniotic leakage from the puncture site; most settle with rest.
• Infection (chorioamnionitis) is uncommon but serious: fever, uterine tenderness, offensive discharge, or a raised white cell count. Exclude infection before resuming routine care.
• Rupture of membranes before 28 weeks: watery leaking after the procedure requires ultrasound (amniotic fluid index) and hospital review.
• Failed sample or laboratory failure. Repeat testing or switch routes; laboratory failure was higher after early amniocentesis.
• Wrong or uninterpretable results, including confined placental mosaicism after CVS (1 to 2 percent), requiring confirmatory amniocentesis.
• Fetal injury is very rare with ultrasound guidance.
• Limb defects after very early CVS remain a reason to avoid CVS before 10 weeks.

• RCOG Green-top Guideline No. 8, 5th edition (2021, reviewed 2024). Women should be informed that the added risk of miscarriage after amniocentesis or CVS by a skilled operator is likely below 0.5 percent, rising to around 1 percent in a twin pregnancy. Amniocentesis should not be done before 15+0 weeks; CVS is done at 11+0 to 13+6 weeks and not before 10+0 weeks. Women considering these tests should receive detailed counselling and pregnancy mapping by trained professionals, and blood-borne virus results, viral load and antigen tests should be reviewed with individualised transmission risk discussed.^[1]
• ACOG Practice Bulletin No. 162 (2016). Prenatal diagnostic testing is done to determine, with as much certainty as possible, whether a specific genetic disorder is present in the fetus, and amniocentesis or CVS is needed to diagnose most genetic disorders definitively.^[2]
• NICE guideline NG201 (2021, last reviewed 2024). Booking by 10+0 weeks; 10 routine appointments for nulliparous women and 7 for parous women; blood pressure and urine protein at every face-to-face visit; an anomaly scan at 18+0 to 20+6 weeks and a dating scan at 11+2 to 14+1 weeks; screening programmes (infectious diseases in pregnancy, sickle cell and thalassaemia, fetal anomaly) offered at booking with the right to decline any part.^[3]
• WHO 2016 recommendations on antenatal care, with the 2022 imaging update. A minimum of eight contacts, first within the first 12 weeks then at 20, 26, 30, 34, 36, 38 and 40 weeks; eight or more contacts reduce perinatal deaths by up to 8 per 1000 births compared with four visits; one ultrasound scan before 24 weeks to estimate gestational age, improve detection of fetal anomalies and multiple pregnancies and reduce labour induction for post-term pregnancy; daily oral iron 30 to 60 mg elemental iron plus 400 micrograms (0.4 mg) folic acid to prevent maternal anaemia, puerperal sepsis, low birth weight and preterm birth.^[4]
• CDC guidance (updated 2024). All pregnant women should be tested for HIV, hepatitis B, hepatitis C and syphilis in every pregnancy, with additional timed testing for chlamydia and gonorrhoea in younger women or those at risk.^[5]
• Cochrane review of randomised trials (2017). Second-trimester amniocentesis is the procedure of first choice from 15 weeks onwards. From 11 weeks, transabdominal CVS is generally preferred to early amniocentesis, which gave more losses and more talipes. Diagnostic accuracy could not be fully assessed because karyotype data in the trials were incomplete.^[6]

The decision flow below is a teaching outline, not a protocol; follow local guidance and involve the fetal medicine team.

Routine antenatal care frames everything else. NICE sets the common European standard used here; WHO frames care as person-centred and available in any setting.^[3][4]

• Booking by 10+0 weeks with a midwife (NICE): history, domestic-abuse screening in a private setting, medicines (including herbal and over-the-counter), allergies, occupation, nutrition, smoking and alcohol, mental-health screen, and a review of previous records.
• Routine appointment counts: 10 for a first pregnancy, 7 for later pregnancies, with more if medical or social needs require them.
• First face-to-face visit: measure height, weight and body mass index; take blood for full blood count, blood group and rhesus D status.
• Scans: a dating scan at 11+2 to 14+1 weeks and an anomaly scan at 18+0 to 20+6 weeks to screen for fetal anomalies and locate the placenta.
• 28 weeks: full blood count, blood group and antibodies, and anti-D prophylaxis for rhesus-D-negative women who are not already sensitised.
• Blood pressure and urine protein are recorded at every routine face-to-face visit. Risk of venous thromboembolism is assessed at booking and after any admission or significant event.
• Common-problem screening: women at risk of gestational diabetes are referred for an oral glucose tolerance test at 24+0 to 28+0 weeks. ^[3]
• Fetal assessment after 24 weeks: symphysis-fundal height at least every 2 weeks for singletons (unless regular growth scans are done), questions about reduced fetal movements, and palpation for breech after 36 weeks. ^[3]
• WHO model: a minimum of eight contacts, first within the first 12 weeks then at 20, 26, 30, 34, 36, 38 and 40 weeks, reduces perinatal deaths by up to 8 per 1000 births compared with four visits. Counsel about healthy diet, physical activity, tobacco and substance use (including asking about alcohol and substances as early as possible and at every visit), and availability of continuing care. Daily oral iron 30 to 60 mg elemental iron with 400 micrograms (0.4 mg) folic acid daily is recommended to prevent maternal anaemia. One ultrasound before 24 weeks is recommended for dating and anomaly detection.^[4]
• Immunisation: flu, pertussis (whooping cough), respiratory syncytial virus and other infections in pregnancy per national immunisation policy.

Screening practices vary by country, but the common set is consistent across the guidelines:

• Universal testing (CDC): HIV, hepatitis B (HBsAg), hepatitis C and syphilis in every pregnancy. Testing is ideally early, with further timed testing for certain groups: syphilis at 28 weeks, HIV before 36 weeks, and repeat HIV, HBV and HCV testing at delivery if earlier screening was missed.^[5]
• NICE screening programme: HIV, syphilis and hepatitis B are offered at the booking appointment, alongside blood tests for haemoglobinopathies (sickle cell and thalassaemia). The person has the right to accept or decline any part.^[3]
• Additional testing by risk (CDC): chlamydia and gonorrhoea for all women under 25 and older women at risk (new or multiple partners, partner with concurrent partners, partner with an STI), with third-trimester repeat testing for those at continuing risk; targeted testing for tuberculosis in women with risk factors (migration from a high-prevalence country, HIV, diabetes, immunosuppression, or contact with a case); HBsAg testing for any woman not screened previously, those at high risk or with signs or symptoms of hepatitis.^[5]
• Antenatal information (NICE): women are told about infections that can affect the baby (group B streptococcus, herpes simplex and cytomegalovirus) and about simple preventive measures such as hand hygiene. NICE does not recommend routine screening for these; symptomatic women should be investigated. Group B streptococcus carriage is the usual reason for intrapartum antibiotic prophylaxis in risk-based protocols.^[3]
• Other infections discussed in antenatal care: WHO highlights malaria and HIV prevention and counselling for women experiencing intimate partner violence as part of the 2016 model. Clinicians follow national schedules (for example rubella immunity, varicella and syphilis) where the local programme includes them. Invasive procedures should not be delayed indefinitely, but if an infection is suspected, treat first.
• Why this matters for invasive testing: if infection is suspected, maternal infection may affect both the result and the fetus. Before an invasive test, review all blood-borne virus results, viral load and antigen tests and discuss individualised transmission risk.^[1]

Classic TORCH (toxoplasmosis, rubella, cytomegalovirus, herpes and other infections) is a teaching grouping of perinatal infections; screening is selective rather than universal for most of its members.

• Multiple pregnancy. Added miscarriage risk after CVS or amniocentesis by a skilled operator is around 1 percent.^[1] Non-invasive testing has more limits in twins; diagnostic testing remains an option from the appropriate gestation.
• Women living with HIV or hepatitis B or C. Individualise timing and route, review viral load, and consider the risk of vertical transmission. Antepartum care also includes linkage to treatment.^[1][5]
• Late presentation. A woman who books late should be offered urgent assessment; if the gestation is already beyond 15 weeks and there is an indication, amniocentesis can still be arranged.
• Previous invasive test in a prior pregnancy. Routine; no special contraindication beyond the usual.

• Most deaths prevented by antenatal care come from simple measures: booking early, blood pressure and urine protein every visit, timely referral for danger signs, one ultrasound before 24 weeks, and iron and folate supplementation.^[4]
• Where CVS or amniocentesis is not available, women should still be counselled, offered screening where possible, and referred regionally when an indication arises.
• If a procedure is being done in a smaller centre, keep to the gestational-age windows, use ultrasound guidance, use a single careful pass where possible, and arrange same-day review and clear routes for complications.
• Blood-borne virus screening is the single most important infectious-disease safeguard before any invasive test.

• CVS and amniocentesis are diagnostic tests that are offered, not imposed; informed consent is central.
• CVS is done at 11+0 to 13+6 weeks and should not be done before 10+0 weeks; amniocentesis is done from 15+0 weeks.
• Early amniocentesis (before 15 weeks) is not a safe alternative: more losses, more spontaneous miscarriage and more talipes.
• In skilled hands the added miscarriage risk is probably below 0.5 percent (about 1 percent in twins).
• Transabdominal CVS shows no clear difference in pregnancy loss compared with second-trimester amniocentesis; transcervical CVS may carry a higher loss and is more likely to fail.
• Confined placental mosaicism (1 to 2 percent of CVS) means a mosaic CVS result needs confirmatory amniocentesis.
• Before an invasive test, review blood-borne virus results and discuss individualised transmission risk.
• Routine antenatal care (NICE/WHO) surrounds the procedure: early booking, regular blood pressure and urine protein, dating and anomaly scans, infection screening and iron/folate.
• Universal infection screening in pregnancy covers HIV, hepatitis B, hepatitis C and syphilis; chlamydia and gonorrhoea are added for younger women or those at risk.
• Screening is not diagnosis; confirm any high-risk screen before acting.
• RhD-negative women receive anti-D prophylaxis after the procedure, following the local protocol.

Invasive prenatal testing sits alongside prenatal screening, fetal anomaly ultrasound, the common trisomies (Down, Edwards, Patau syndromes), genetic counselling, termination of pregnancy, multiple pregnancy, women's health in pregnancy and routine antenatal care. These are written as plain-text cross-references; link targets are added when the corresponding MedGrasp topics are verified.