Human Anatomy
Prenatal Diagnosis
Prenatal diagnosis for MBBS: ultrasound, maternal serum screening, amniocentesis and chorionic villus sampling with indications and risks, mapped to NMC codes AN81.1 to AN81.3.
MedNext Academy | 3 min read
Prenatal Diagnosis
Prenatal diagnosis for MBBS: ultrasound, maternal serum screening, amniocentesis and chorionic villus sampling with indications and risks, mapped to NMC codes AN81.1 to AN81.3.
This chapter describes the non-invasive and invasive methods of prenatal diagnosis, including ultrasound, maternal serum screening, amniocentesis and chorionic villus sampling. It covers the indications, technique and drawbacks of the main invasive tests.
High-yield: Prenatal Diagnosis
- Prenatal diagnosis aims to detect fetal abnormalities before birth to guide counselling and management.
- Methods are broadly non-invasive, such as ultrasound and maternal blood tests, and invasive, such as amniocentesis and chorionic villus sampling.
- Ultrasound is the mainstay of non-invasive screening and assesses fetal growth, anatomy and gestational age.
- Nuchal translucency measured by ultrasound in the first trimester screens for chromosomal abnormalities.
- Maternal serum alpha-fetoprotein is raised in open neural tube defects and low in Down syndrome.
- The triple and quadruple tests combine maternal serum markers to estimate the risk of chromosomal disorders.
- Amniocentesis samples the amniotic fluid, usually after the fifteenth week, to obtain fetal cells for karyotyping.
- Amniocentesis carries a small risk of miscarriage and, if done too early, of limb defects.
- Chorionic villus sampling takes a sample of placental tissue, usually between the tenth and thirteenth weeks.
- Chorionic villus sampling gives an earlier result than amniocentesis but carries a slightly higher risk of miscarriage.
- Both invasive tests provide fetal cells for chromosomal, biochemical and molecular analysis.
- Cordocentesis samples fetal blood directly from the umbilical cord for rapid analysis.
- Non-invasive prenatal testing analyses cell-free fetal DNA in the maternal blood to screen for common trisomies.
- A raised maternal serum alpha-fetoprotein prompts a detailed ultrasound to look for a neural tube defect.
- Prenatal diagnosis is combined with genetic counselling so that parents can make informed decisions.
Prenatal diagnostic methods
- **Ultrasound:** Non-invasive; assesses growth, anatomy and age. Nuchal translucency screens for chromosomal disorders.
- **Maternal serum markers:** Alpha-fetoprotein raised in open neural tube defects, low in Down syndrome; part of triple and quadruple tests.
- **Amniocentesis:** After the fifteenth week; fetal cells from amniotic fluid for karyotyping. Small miscarriage risk.
- **Chorionic villus sampling:** Weeks ten to thirteen; placental tissue for earlier results. Slightly higher miscarriage risk.
NMC competencies in this chapter
- **AN81.1:** Methods of prenatal diagnosis
- **AN81.2:** Indications, process and disadvantages of amniocentesis
- **AN81.3:** Indications, process and disadvantages of chorionic villus biopsy
Frequently Asked Questions
What is the difference between amniocentesis and chorionic villus sampling?
Amniocentesis samples amniotic fluid, usually after the fifteenth week, while chorionic villus sampling takes placental tissue between the tenth and thirteenth weeks. Chorionic villus sampling gives an earlier result but carries a slightly higher miscarriage risk.
What does maternal serum alpha-fetoprotein indicate?
A raised level suggests an open neural tube defect such as spina bifida or anencephaly, while a low level is associated with Down syndrome. An abnormal result prompts detailed ultrasound.
What are the non-invasive methods of prenatal diagnosis?
Ultrasound, maternal serum screening with markers such as alpha-fetoprotein, and non-invasive prenatal testing that analyses cell-free fetal DNA in the mother's blood.
What are the risks of invasive prenatal tests?
Both amniocentesis and chorionic villus sampling carry a small risk of miscarriage. Very early amniocentesis has been linked with limb defects, and chorionic villus sampling has a slightly higher procedure-related loss rate.
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