A chemical pregnancy is a confusing term for an early miscarriage in which the pregnancy stops developing before an ultrasound would show a gestational sac, thus meaning that the pregnancy can only be proven by checking hCG levels.
In a normally developing pregnancy, a transvaginal ultrasound would show a gestational sac by about 5 weeks after the last menstrual period, assuming a 28-day cycle. So, pregnancies that stop developing before about 5 weeks of gestational age would be considered chemical pregnancies. It is possible for a chemical pregnancy to draw out slightly longer than 5 weeks, however, if the pregnancy has stopped developing but the miscarriage bleeding doesn't begin right away.
Source:
Pagidas, Kelly and Lucia Larson. Medical Care of the Pregnant Patient. ACP Press, 2007. Accessed 1 Dec 2008
