Carbon dioxide is a pure substance. A pure substance can be an element or a compound. Carbon dioxide is a pure compound, consisting of two kinds of atoms that are bonded together. An element consists of merely one kind of atom. For example, oxygen is an element.
For Downâs syndrome, a blood test called the quadruple test is available from weeks 14-20 of pregnancy. For Patauâs and Edwardsâ syndromes, you will be offered a mid-pregnancy scan that checks for physical abnormalities.
In general, however, if the hCG levels are dropping in the first trimester, this probably a sign of impending miscarriage. On the other hand, slow-rising hCG levels that do not double every two or three days in early pregnancy can be a sign of problems but can also result in a normal pregnancy.