Tests...what and why? DIAGNOSTIC TESTS. These show conclusively whether a baby has certain chromosomal disorders,such as downs syndrome,spina bifida or anencephaly. DIAGNOSTIC TESTS ARE USUALLY ONLY OFFERED TO WOMEN OVER 35. ALTHOUGH THESE TESTS ARE ALMOST 100% ACCURATE THEY CARRY A SMALL RISK OF MISCARRIAGE. AMNIOCENTESIS 16/18 WEEKS ULTRASOUND IS USED AS A GUIDE TO REMOVE, VIA A NEEDLE THROUGH THE ABDOMEN, A SMALL AMOUNT OF AMNIOTIC FLUID SURROUNDING BABY, INTO WHICH IT SHEDS SKIN CELLS THESE CELLS ARE CULTURED THEN TESTED. RESULTS CAN TAKE 2/4 WEEKS BY WHICH TIME YOU MAY BE 22 WEEKS PREGNANT. MISCARRIAGE RISK IS APROX 1% CORDIOCENTESIS AFTER 20 WEEKS, CORDOCENTESIS IS USED TO TAKE BLOOD DIRECTLY FROM BABY VIA THE UMBLICAL CORD. iT IS THE QUICKEST DIAGNOSTIC TEST , WITH RESULTS AVAILABLE IN ONE TO SEVEN DAYS IN CERTAIN HOSPITALS. MISCARRIAGE RATE IS UP TO 2 PERCENT. CVS CARRIED OUT EARLY AT APROX 11/13 WEEKS. TECHNICALLY COMPLICATED. TINY PIECE OF PLACENTA IS RETRIEVED BY NEEDLE INSERTED THROUGH THE VAGINA AND CERVIX OR ABDOMEN. MISCARRIAGE RATE IS ABOUT 3% OVERALL BUT 1% IN MORE EXPERIANCED CENTRES. LOWER RATES APPEAR TO BE WITH ABDOMINAL ROUTE. SCREENING TESTS. AFP :LEAST ACCURATE BLOOD TEST ON OFFER. LOW LEVELS MAY INDICATE INCREASED RISK OF DOWNS SYNDROME. HIGH LEVELS SPINA BIFIDA. DOUBLE/TRIPLE/QUADRUPLE. MEASURE AFP AND ONE TWO OR THREE OTHER SUBSTANCES IN THE BLOOD TO INCREASE RELIABILITY THESE TESTS ARE NOT CONCLUSIVE. NUCHAL TEST EARLIEST TEST AT BETWEEN 10/14 WEEKS. MEASURES FLUID FILLED SPACE AT BACK OF BABYS NECK ULTRASOUND WIDELY AVAILABLE SCAN ALMOST ROUTINELY OFFERED. TO DATE THE BABY AND TO LOOK FOR ABNORMALITIES. GEL IS SMEARED OVER TUMMY AND A SCANNING DEVICE PASSED OVER IT WHICH SENDS OUT AND RECIEVES SOUND WAVES. FROM THESE THE COMPUTER BUILDS UP A PICTURE SHOWN ON THE SCREEN. MARKER SCAN A DETAILED SCAN WHICH MOST HOSPITALS OFFER AT 20 WEEKS. TINY MARKERS OF POSSIBLE CHROMOSOMAL PROBLEMS ARE LOOKED FOR SUCH AS THE SIZE OF THE GAP BETWEEN THE FIRST AND SECOND TOE. All of these tests are optional, they are not compulsory and you can refuse them if you do not wish to have them.