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CVS and Amniocentesis

Two tests that can give a clear answer about some genetic conditions

CVS and amniocentesis are diagnostic tests. They collect cells from the pregnancy so a lab can test for some chromosome and gene conditions.

A screening test tells the chance of a condition. A diagnostic test looks for the condition itself.

Illustration comparing CVS, which takes a small sample from the placenta, with amniocentesis, which takes a small sample of the fluid around the baby.
In CVS, the needle enters the placenta to collect a small placenta sample. In amniocentesis, the needle enters the amniotic sac to collect a small amount of fluid around the baby.
Both tests use ultrasound for guidance. The sample can be used for tests such as karyotype, microarray, or a targeted gene test. The lab test must match the question being asked.

CVS - Chorionic Villus Sampling

Usual timing: 11 weeks 0 days to 13 weeks 6 days.

Sometimes: It may be done from 14 weeks 0 days to 14 weeks 6 days if needed.

Do not do before: 10 weeks 0 days.

Sample: A small piece of the placenta.

How: Usually a thin needle goes through the belly. Sometimes a thin tube goes through the cervix.

Advantages
  • Can be done earlier in pregnancy.
  • Results may come earlier.
  • Can test chromosomes and many known gene conditions.
  • Can give families more time to learn and plan.
Limits
  • The sample comes from the placenta, not the fluid around the baby.
  • Rarely, the placenta may have a chromosome change that the baby does not have. This is called confined placental mosaicism.
  • If the result is unclear, an amniocentesis may still be needed.
  • CVS does not test the amniotic fluid for AFP.

Amniocentesis ("Amnio")

Usual timing: From 15 weeks 0 days and later.

Often done: Between 15 and 20 weeks, but it can also be done later.

Sample: A small amount of the fluid around the baby.

How: A thin needle goes through the belly into the fluid while ultrasound guides the needle.

Advantages
  • Tests cells found in the fluid around the baby.
  • Can test chromosomes and many known gene conditions.
  • Can also test the fluid for AFP when a brain or spine opening is a concern.
  • Less likely than CVS to be confusing because of a placenta-only result.
Limits
  • It is done later than CVS.
  • You may wait longer to get the answer.
  • No test checks for every condition.

When might one test be used?

I want an earlier answer: CVS may be the better choice.
I am 15 weeks or later: Amniocentesis is available and may be a good option.
A known family gene condition is being tested: Either test may work if the lab does the right genetic test.
A brain or spine opening is a concern: Amniocentesis may help because the fluid can be tested for AFP.
The placenta is hard to reach: CVS may not be possible, and amniocentesis later may be recommended.
I am between 14 weeks 0 days and 14 weeks 6 days: Your doctor may help choose whether CVS or amniocentesis fits best.
Remember: CVS and amniocentesis collect a sample. The lab test done on that sample must match the question your care team is trying to answer.
Medical references used for this handout
  1. Navaratnam K, Alfirevic Z; Royal College of Obstetricians and Gynaecologists. Amniocentesis and chorionic villus sampling: Green-top Guideline No. 8. BJOG 2022;129:e1-e15.
  2. RCOG Green-top Guideline No. 8, October 2021. CVS is usually performed between 11+0 and 13+6 weeks; if needed, 14+0 to 14+6 weeks. Amniocentesis is usually offered from 15+0 weeks.
  3. American College of Obstetricians and Gynecologists. Prenatal Genetic Diagnostic Tests. Patient FAQ.