Some screening tests performed on the woman are intended to detect traits or characteristics of the fetus.Others detect conditions in the woman that may have an adverse effect on the fetus, or that threaten the pregnancy.The screening focuses on finding problems among a large population with affordable and noninvasive methods, whereas the diagnosis focuses on pursuing additional detailed information once a particular problem has been found, and can sometimes be more invasive.Screening can also be used for prenatal sex discernment.However, transcervical chorionic villus sampling carries a signiﬁcantly higher risk, compared with a second trimester amniocentesis, of total pregnancy loss (relative risk 1.40; 95% confidence interval 1.09 to 1.81) and spontaneous miscarriage (9.4% risk; relative risk 1.50; 95% confidence interval 1.07 to 2.11).Non-invasive techniques include examinations of the woman's womb through ultrasonography and maternal serum screens (i.e. Blood tests for select trisomies (Down syndrome in the United States, Down and Edwards syndromes in China) based on detecting cell-free placental DNA present in maternal blood, also known as non-invasive prenatal testing (NIPT), have become available.
An invasive method involves probes or needles being inserted into the uterus, e.g.
For example, abnormally low levels of the serum marker PAPP-A have been shown to correspond to an increased risk of pre-eclampsia, in which the mother's high blood pressure can threaten the pregnancy, though many physicians find regular blood-pressure monitoring to be more reliable.
Prenatal diagnosis and prenatal screening are aspects of prenatal care that focus on detecting anatomic and physiologic problems with the zygote, embryo, or fetus as early as possible, either before gestation even starts (as in preimplantation genetic diagnosis) or as early in gestation as practicable.
They use medical tests to detect problems such as neural tube defects, chromosome abnormalities, and gene mutations that would lead to genetic disorders and birth defects, such as spina bifida, cleft palate, Tay–Sachs disease, sickle cell anemia, thalassemia, cystic fibrosis, muscular dystrophy, and fragile X syndrome.
amniocentesis, which can be done from about 14 weeks gestation, and usually up to about 20 weeks, and chorionic villus sampling, which can be done earlier (between 9.5 and 12.5 weeks gestation) but which may be slightly more risky to the fetus.
One study comparing transabdominal chorionic villus sampling with second trimester amniocentesis found no signiﬁcant difference in the total pregnancy loss between the two procedures.