Prenatal genetic testing and multiple pregnancies

Hands holding a scan image of twin eggs

Expecting twins, triplets or higher multiples affects the genetic test options in a pregnancy. Parents in these circumstances should be referred to fetal medicine specialists, who will be able to advise regarding options.

1. Prenatal genetic screening in twin pregnancies

It is not always possible to be sure in pregnancy whether twins are genetically identical. However, it is important that a doctor or sonographer uses ultrasound to look carefully at how the membranes or ‘barriers’ around the baby or babies are arranged, and whether they share a placenta. This will be helpful in planning how to manage the pregnancy and in considering genetic testing.

For identical twins, screening tests should work in a similar way as they do for singleton pregnancies because both babies have the same genetic make-up. In non-identical twins, screening is more challenging and is usually less accurate than in singleton pregnancies. For example, if you have a blood test to measure hormone levels or cell-free DNA, it will be difficult to give an accurate result for each baby. So, ultrasound markers such as the nuchal translucency (pocket of fluid at the back of the baby’s neck measured as part of combined screening for Down’s syndrome, Edwards’ syndrome and Patau’s syndrome) are more useful as this can be measured for each baby.

Some couples expecting twins decide to have genetic screening for information purposes, to prepare themselves and their family for the possibility of having one or more baby with a disability. It can be helpful to discuss and consider potential outcomes before opting for screening as testing in twin pregnancies is more complex than when there is one baby.

2. Prenatal Genetic Diagnosis in twin pregnancies

CVS and amnio carry a slightly higher risk in twin pregnancies and should always be done by an experienced fetal medicine specialist. If one twin is thought to have a higher chance of having a genetic condition, it will be important that the specialist ‘maps’ the pregnancy carefully so it is clear which twin this is.

The same kind of analysis can be done on the sample as for singleton pregnancies. If one twin is found to have a genetic condition, some woman and couples decide to have a selective reduction and terminate the baby with the condition. Specialists will discuss when and how this might be carried out in order to maximise the chances for the twin who doesn’t have the genetic condition.