First trimester screening is a set of tests done early in pregnancy, usually between 11 and 14 weeks, to estimate the chance that the baby has certain chromosome conditions.
Chromosomes are the packages that carry our DNA. Most people have 46 chromosomes in each cell, arranged in 23 pairs. Sometimes a baby has an extra copy of one chromosome, which is called a trisomy. First trimester screening looks mainly for three of these:

• Down syndrome (trisomy 21) An extra copy of chromosome 21. It is the most common chromosome condition in babies born alive. It causes learning disability of varying degree, and some babies have heart or bowel differences.
• Edwards syndrome (trisomy 18) An extra copy of chromosome 18. It causes serious problems with the heart and other organs, and many pregnancies do not continue to birth.
• Patau syndrome (trisomy 13) An extra copy of chromosome 13. Like Edwards syndrome, it is usually very serious.

Any pregnancy can be affected. These changes usually happen by chance when the egg or sperm is formed, and not because of anything the parents did. The chance does rise with the mother’s age. At age 20, the chance of Down syndrome at birth is about 1-in-1,250. At age 40, it is about 1-in-86. This is why professional guidelines, including those of the American College of Obstetricians and Gynaecologists, recommend offering screening to every pregnant person, whatever their age.

The scan part of the test can also give early clues about other conditions, such as Turner syndrome (a missing X chromosome in a girl), triploidy (a full extra set of chromosomes), and some heart and skeletal conditions.

It is important to know what first trimester screening is not. It is a screening test, not a diagnosis. It gives a chance, not a yes or no answer. It also does not check for single-gene conditions such as beta-thalassemia, sickle cell disease, or spinal muscular atrophy. Those need separate carrier screening of the parents.