First-trimester screening

Around this stage of your pregnancy, you will have one of the most important tests of the first trimester: the combined screening.

To carry out this screening, three pieces of information are combined: your age, the results of a blood test done between 10 weeks and 13 weeks + 6 days, and the data from your first-trimester scan, performed between 11 weeks and 13 weeks + 6 days, which assesses your baby's development, the gestational age, and some important ultrasound markers.

One of the greatest difficulties of this test is understanding the meaning of the word "probability".

Imagine you are about to leave the house and you check the weather forecast. The app says there is a 20% probability of rain. Does this mean it is going to rain? No. It simply means that, taking into account all the available information, there is a certain probability of it happening. The first-trimester combined screening works in a very similar way. It is a test that uses different pieces of information about your pregnancy to calculate the probability of the baby having certain chromosomal changes.

This screening does not give a diagnosis. A low-risk result means that the probability is low. A higher-probability result does not mean the baby has a chromosomal change; it simply means that it may be helpful to carry out more specific tests to clarify this possibility.