<Li> Women who have previously had miscarriages </Li> <P> Diagnostic prenatal testing can be performed by invasive or non-invasive methods . An invasive method involves probes or needles being inserted into the uterus, e.g. 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 . 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). </P> <P> Non-invasive techniques include examinations of the woman's womb through ultrasonography and maternal serum screens (i.e. Alpha - fetoprotein). 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 . If an elevated risk of chromosomal or genetic abnormality is indicated by a non-invasive screening test, a more invasive technique may be employed to gather more information . In the case of neural tube defects, a detailed ultrasound can non-invasively provide a definitive diagnosis . </P> <Table> <Tr> <Td> Invasiveness </Td> <Td> Test </Td> <Td> Comments </Td> <Td> Time </Td> </Tr> <Tr> <Td> Non-invasive </Td> <Td> Preimplantation genetic diagnosis (PGD) </Td> <Td> During in vitro fertilization (IVF) procedures, it is possible to sample cells from human embryos before implantation . PGD is in itself non-invasive, but IVF usually involves invasive procedures such as transvaginal oocyte retrieval </Td> <Td> before implantation </Td> </Tr> <Tr> <Td> Non-invasive </Td> <Td> External examination </Td> <Td> Examination of the woman's uterus from outside the body . The uterus is commonly palpated to determine if there are problems with the position of the fetus (i.e. breech position). Fundal height may also be measured . </Td> <Td> Second or third trimester </Td> </Tr> <Tr> <Td> Non-invasive </Td> <Td> Ultrasound detection </Td> <Td> Commonly dating scans (sometimes known as booking scans or dating ultrasounds) from 7 weeks to confirm pregnancy dates and look for multiple pregnancies . The specialised nuchal scan at 11--13 weeks may be used to identify higher risks of Downs syndrome . Later morphology scans, also called anatomy ultrasound, from 18 weeks may check for any abnormal development . Additional ultrasounds may be performed if there are any other problems with the pregnancy, or if the pregnancy is post-due . </Td> <Td> First or second trimester </Td> </Tr> <Tr> <Td> Non-invasive </Td> <Td> Fetal heartbeat </Td> <Td> Listening to the fetal heartbeat via an external monitor placed on the outside of the abdomen . </Td> <Td> First or second trimester </Td> </Tr> <Tr> <Td> Non-invasive </Td> <Td> Non-stress test </Td> <Td> Use of cardiotocography during the third trimester to monitor fetal wellbeing . </Td> <Td> Third trimester </Td> </Tr> <Tr> <Td> Non-invasive </Td> <Td> Maternal blood pressure </Td> <Td> Used to screen for pre-eclampsia throughout the pregnancy . </Td> <Td> First, Second and Third trimester </Td> </Tr> <Tr> <Td> Non-invasive </Td> <Td> Maternal weighing </Td> <Td> Unusually low or high maternal weight can indicate problems with the pregnancy . </Td> <Td> First, Second and Third trimesters . </Td> </Tr> <Tr> <Td> Less invasive </Td> <Td> Fetal cells in maternal blood (FCMB) </Td> <Td> Requires a maternal blood draw . Based on enrichment of fetal cells which circulate in maternal blood . Since fetal cells hold all the genetic information of the developing fetus, they can be used to perform prenatal diagnosis . </Td> <Td> First trimester </Td> </Tr> <Tr> <Td> Less invasive </Td> <Td> Cell - free fetal DNA in maternal blood </Td> <Td> Requires a maternal blood draw . Based on DNA of fetal origin circulating in the maternal blood . Testing can potentially identify fetal aneuploidy (available in the United States, beginning 2011) and gender of a fetus as early as six weeks into a pregnancy . Fetal DNA ranges from about 2--10% of the total DNA in maternal blood . <P> Cell - free fetal DNA also allows whole genome sequencing of the fetus, thus determining the complete DNA sequence of every gene . </P> </Td> <Td> First trimester </Td> </Tr> <Tr> <Td> Less invasive </Td> <Td> Glucose tolerance testing </Td> <Td> Requires a maternal blood draw . Used to screen for gestational diabetes . </Td> <Td> Second trimester </Td> </Tr> <Tr> <Td> Less invasive </Td> <Td> Transcervical retrieval of trophoblast cells </Td> <Td> Cervical mucus aspiration, cervical swabbing, and cervical or intrauterine lavage can be used to retrieve trophoblast cells for diagnostic purposes, including prenatal genetic analysis . Success rates for retrieving fetal trophoblast cells vary from 40% to 90% . It can be used for fetal sex determination and identify aneuploidies . Antibody markers have proven useful to select trophoblast cells for genetic analysis and to demonstrate that the abundance of recoverable trophoblast cells diminishes in abnormal gestations, such as in ectopic pregnancy or anembryonic gestation . </Td> <Td> First trimester </Td> </Tr> <Tr> <Td> Less invasive </Td> <Td> Maternal serum screening </Td> <Td> Including β - hCG, PAPP - A, alpha fetoprotein, inhibin - A . See separate section below </Td> <Td> First or second trimester </Td> </Tr> <Tr> <Td> More invasive </Td> <Td> Chorionic villus sampling </Td> <Td> Involves getting a sample of the chorionic villus and testing it . This can be done earlier than amniocentesis, but may have a higher risk of miscarriage, estimated at 1% . </Td> <Td> After 10 weeks </Td> </Tr> <Tr> <Td> More invasive </Td> <Td> Amniocentesis </Td> <Td> This can be done once enough amniotic fluid has developed to sample . Cells from the fetus will be floating in this fluid, and can be separated and tested . Miscarriage risk of amniocentesis is commonly quoted as 0.06% (1: 1600). By amniocentesis ut is also possible to cryopreserve amniotic stem cells . </Td> <Td> After 15 weeks </Td> </Tr> <Tr> <Td> More invasive </Td> <Td> Embryoscopy and fetoscopy </Td> <Td> Though rarely done, these involve putting a probe into a women's uterus to observe (with a video camera), or to sample blood or tissue from the embryo or fetus . </Td> <Td> </Td> </Tr> <Tr> <Td> More invasive </Td> <Td> Percutaneous umbilical cord blood sampling </Td> <Td> PUBS is a diagnostic genetic test that examines blood from the fetal umbilical cord to detect fetal abnormalities . </Td> <Td> 24--34 weeks </Td> </Tr> </Table>

Which tests are noninvasive and performed to detect chromosomal abnormalities in the fetus