The second trimester screening tests include a morphology scan, usually done between 18-22 weeks, and blood tests at 28 weeks which include a full blood count, blood group and antibodies and a glucose challenge test.
The morphology scan is discussed in more detail elsewhere on this website, but essentially it is a test looking at fetal anatomy and placental location. It is ideally performed at 20 weeks gestation.
Ideally you should see your Obstetrician shortly after this test so that the results can be explained. If there are any abnormalities these may need further tests.
Common abnormalities are problems with placental location which may require a further ultrasound scan at approximately 32-34 weeks, or there may be a problem with the fetal anatomy which may require more specialised scans and referral to a maternal fetal medicine specialist or a fetal cardiologist.
The blood tests performed at 28 weeks include the full blood count, blood group and antibodies and a 2-hour glucose tolerance test. Antibodies are only checked if the mother is rhesus negative. Screening for gestational diabetes usually requires a one hour glucose challenge test.
I prefer to perform the diagnostic test up front. This is a two hour glucose tolerance test with a 75g glucose load.
My reasoning for this is that diagnosis is reached at an earlier time and treatment can be started at the earliest time to limit problems in pregnancy.
Your Obstetrician would generally tell you how to prepare for this test as it is a fasting test.