A chemical pregnancy is a pregnancy that has been confirmed by a pregnancy test (either blood or urine), but there are no other significant clinical signs of the pregnancy and there is nothing seen on ultrasound. The urine pregnancy test is often faintly positive or blood hCG levels are very low and don't increase appropriately.
In a chemical pregnancy, it is believed that fertilization occurs, but the egg dies shortly after implantation. Using beta-hCG blood testing, a woman can test positive for pregnancy as early as 1-2 days before a menstrual period is missed, usually 11 or 12 days after conception.
Although the blood test can detect a very low hCG level that indicates pregnancy a few days before a period, two to three days later, a period may start. Without the test, most women would never have known they were briefly pregnant.
A chemical pregnancy happens much more frequently than most people expect, probably up to 30-40% of all pregnancies end up in a chemical pregnancy, and without early testing, most women would never have known about it. If the pregnancy develops and goes unnoticed for several more weeks, a sac may form but without an embryo, which is known as a 'blighted ovum'.