There's a one in two (50%) chance of the baby inheriting the syndrome.
To do this, two possible tests can be used – Chorionic Villus Sampling (CVS) or Amniocentesis.
Chorionic Villus Sampling
Prenatal testing for Marfan syndrome can be carried out approximately 10 to 12 weeks into the pregnancy using chorionic villus sampling (CVS).
CVS involves taking a small sample of cells from the organ that links the mother's blood supply with her unborn baby's (the placenta) through the entrance of the womb. The sample can then be tested for genetic conditions.
Amniocentesis can also be used to test for Marfan syndrome. The test is carried out about 16 to 18 weeks into the pregnancy and involves taking a small sample of amniotic fluid for examination. Amniotic fluid surrounds the unborn baby in the womb.
Although prenatal tests may show whether your child has the defective gene that causes Marfan syndrome, the tests won't give any indication as to how serious their symptoms will be. Generally, the baby will be affected to the same extent as other people in their family.
The severity of Marfan syndrome in the parent is an indication of how severe it will be in the child. Your child may only experience very mild symptoms, despite having the genetic mutation. This is because the expression of the gene can vary, even within the same family.
In some cases, the results of CVS or amniocentesis could be negative, suggesting that your child doesn't have the defective gene. However, your child may have a different genetic mutation that wasn't tested for, but could still cause Marfan syndrome.