A chemical pregnacy is the term used to describe a miscarriage that happens extremely early on in a pregnancy. In the case of a chemical pregnancy, a positive pregnancy test usually occurs before a woman has her monthly menstrual period. 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 miscarriage and a chemical pregnancy happen much more frequently than most people expect. Probably up to 30-40% of all pregnancies end up in a chemical pregnancy and miscarriage, and without testing early on, most women would never know about it. If the pregnancy develops for several more weeks, a sac will form, but without an embryo. This is called a 'blighted ovum.' The earliest a sonogram can show the pregnancy is during the 6th week, about 4+ weeks after ovulation or 2+ weeks after you miss your period. Doing two blood pregnancy tests 2-3 days apart, and showing a more than doubling of the level, decreases the chance that it's a chemical pregnancy.