During the first trimester of pregnancy a combined biochemical and ultrasound screening may be performed to rule out Down syndrome and neural tube defects.
First trimester maternal serum screening can check levels of free chorionic gonadotrophin fraction and the plasma protein associated with pregnancy. Sample may be obtained between 8 and 13 weeks of amenorrhea.
Between 11 and 13 weeks of gestation a transvaginal ultrasound is carried out in order to measure the nuchal translucency (NT) of the embryo.
Computational predictive mode shows the risk for diseases or conditions in a foetus or embryo before it is born, such as Down syndrome, Edwards syndrome and neural tube defects.
First Trimester Combined Test has a sensitivity (i.e. detection rate for abnormalities) of 82-87% and a false-positive rate around 5%.

Transvaginal ultrasound in the first trimester of pregnancy has an important role in the estimation of the risk of chromosomal alterations.
Commonly dating scans (or booking scans) from 7 weeks confirm pregnancy dates and look for twins.
At 11-13 weeks the ultrasound detection identify some parameters such as the length craniocaudal (LCC) of the embryo, the nuchal translucency (NT), the presence of the foetal nasalbone and the ductus venosus. The first trimester ultrasound must also include the study of the basic morphology of the head, trunk and limbs of the embryo.

Foetal ultrasound often plays an important role in prenatal care.
Later morphology scans from 18 weeks may check for any abnormal foetal development.
It includes the biometric data of the foetus, as well as the detailed study of the organs and foetal systems, in accordance with the recommendations emanating from the international scientific societies.

This procedure can be done once enough amniotic fluid has developed to sample.
Cells from the foetus will be floating in this fluid, and can be separated and tested for chromosome study.
Miscarriage risk of amniocentesis is commonly quoted as 0,06%.
By amniocentesis is also possible to cryopreserve amniotic stem cells.
Time to perform an amniocentesis is after 15 weeks of amenorrhea.

Percutaneous umbilical cord blood sampling (PUBS) is a diagnostic genetic test that examines blood from the foetal umbilical cord to detect foetal abnormalities.
The procedure entails some risks, and the operator must have experience in this technique.
Cordocentesis can be performed between 24 and 34 weeks of gestation.

If a screening test indicates a possible problem – or your age, family history or medical history puts you at increased risk of having a baby with a genetic problem – you might consider a more invasive prenatal diagnostic test.
The chorionic biopsy involves getting a sample of the chorionic villus for cytogenetic study.
The access ways are transcervical and transabdominal.
The procedure can be done after 10 weeks of amenorrhoea.
The risk of miscarriage is estimated at 1%.

Due to the detection of foetal cells and DNA circulating in maternal blood, non-invasive diagnosis of foetal aneuploidy is becoming more promising.
The Harmony Prenatal Test is a test made in maternal blood, without any risk to the mother nor the foetus.
Unlike the cordocentesis, amniocentesis, or chorionic biopsy, the Harmony test is a non-invasive test.
It allows the detection of alterations in the X,Y,13,18 and 21 chromosomes.