Chorionic villus sampling, or CVS, is a diagnostic test in which a small sample of placental tissue is taken under ultrasound guidance and analysed. Because placental cells share the baby’s genetic make-up, the result gives a definite answer about chromosomal and many genetic conditions. It is performed either through the abdomen with a needle or through the cervix with a fine tube.

The main difference is timing. CVS is usually done between 11 and 14 weeks and amniocentesis from 15 weeks onwards, so CVS gives an answer several weeks earlier and leaves more time to consider the options. Reported miscarriage risk is broadly similar, at under 1 in 200 for most single pregnancies. Amniocentesis samples amniotic fluid rather than placenta and avoids the occasional confined placental mosaicism that CVS can show.

Cordocentesis, also called fetal blood sampling, takes a small blood sample directly from the umbilical cord under ultrasound guidance. It is used when the baby’s own blood must be examined, for example to confirm severe anaemia, to check the platelet count, to diagnose certain infections, or to obtain a rapid chromosome result. It carries a higher risk than CVS or amniocentesis, so it is reserved for situations where nothing else will answer the question.

Fetal therapy is treatment given to the baby during pregnancy rather than after birth. Established examples include intrauterine blood transfusion for severe fetal anaemia, laser treatment for twin-to-twin transfusion syndrome, drainage of fluid collections in the chest, shunt placement for urinary obstruction, and medicines given to the mother to treat the baby’s heart rhythm. These are specialised procedures done only at centres equipped for them.

Growth restriction means the baby is not growing to its own expected potential, usually because the placenta is not supplying enough oxygen and nutrients. It is different from simply being small, since some babies are constitutionally small and entirely well. Distinguishing the two relies on serial growth measurements and Doppler studies of blood flow, and growth-restricted pregnancies need closer monitoring and careful timing of delivery.

Genetic counselling is a structured discussion about the chance that a condition affects this pregnancy or a future one, and about what testing can and cannot establish. It usually covers family history, what a particular result would and would not mean, the options at each stage, and the implications for other family members. Its purpose is to help a family make its own decision, not to recommend one.

Fetal MRI is used as an addition to ultrasound, most often after about 18 weeks, when ultrasound has found something that needs further definition. It is most valuable for the brain and spinal cord, and also for the chest, neck and abdomen where the anatomy is complex. It does not use ionising radiation. It does not replace ultrasound, which remains the first-line examination in pregnancy.

The previous findings are reviewed, the relevant structures are re-examined, and growth and wellbeing are reassessed. Many findings change with time, so the purpose is to establish whether something has resolved, stayed the same or progressed. The visit should end with a clear statement of the current position, what it means for delivery, and when the next assessment is due.