If a pregnancy has been achieved, seven days after fertilization, the embryo begins to secrete the hormone HCG (human chorionic gonadotropin), and this HCG stimulates the ovary to continue to produce progesterone and estrogen, which are necessary for the maintenance of the lining of the womb. Without continued production of progesterone, the pregnancy could not survive. The embryo begins to make HCG when the pregnancy is first established in the uterus, about seven days after ovulation. After three months the fetus, or rather the fetal placenta, actually makes its own progesterone, and the ovaries are no longer needed for production of hormones. After nine months, the baby is ready to be pushed out of the uterus by the mother during labor.

The presence of HCG only signifies that the embryo has implanted and is the basis for almost all of the routine pregnancy tests. Blood tests for pregnancy really just check for the presence of HCG. If it is present, then the pregnancy test is positive. Pregnancy can even be diagnosed with a simple urine test that the woman can perform herself within fourteen days of egg fertilization. However, the laboratory blood test is more reliable. If it is positive, i.e., there is more than twenty-five units of HCG, it should be repeated two days later to see if the HCG level has increased. Normally the HCG doubles every two days for the first month of pregnancy and reaches astronomic levels. If the HCG does not increase, a miscarriage is very likely, and the pregnancy is referred to as a chemical pregnancy.

If the HCG level goes up as it should, then an ultrasound five weeks after fertilization (defined as a seven-week gestational-age pregnancy) should show a normal fetal heartbeat. If there is no fetal heartbeat by seven weeks’ gestation, the pregnancy is not viable and miscarriage will follow. Thus, a positive pregnancy test alone does not ensure that the pregnancy is viable. 

When you have a positive pregnancy test (which just means an elevated HCG level), the chances are 85 percent that you will have a favorable ultrasound at seven weeks and deliver a healthy baby.