A chemical pregnancy is first suspected if the hCG level on the first pregnancy test is positive but too low. A frequent second sign of chemical pregnancy is when the hCG level does not double within two days. Once hCG levels plummet, it becomes very clear that the pregnancy is not normal.
That, however, does not yet necessarily mean that this pregnancy is, or will be, a chemical pregnancy. It is considered as such only if the abnormal pregnancy exists within the uterine cavity. As noted above, if this pregnancy is located outside of the uterus, it is generally considered an ectopic or extrauterine pregnancy rather than a chemical pregnancy. And during these early days of pregnancy when hCG levels are normally low, it is literally impossible to differentiate between early intrauterine and early ectopic pregnancies.
The difficulty in differentiating between these two diagnoses is further aggravated by the fact that early ectopic/tubal pregnancies can also self-terminate, in a process called tubal abortion. As a consequence, a transient rise in hCG during these early days of a pregnancy when one cannot yet expect to see a pregnancy on ultrasound (i.e., before ca. 1500-2000IU/mL of hCG) may be either a chemical pregnancy (in the uterus) or a tubal abortion and an ultimate differential diagnosis may never become clinically possible. [CHR’s Medical Director and Chief Scientist, Norbert Gleicher, MD in 1992 actually published a study, demonstrating that a differential diagnosis can be made radiographically, Fertil Steril 1992;57(3): 553-558]. But since both conditions are self-limited and do not require further follow-up, once hCG levels have returned to being negative, a differential diagnosis is often not of importance and such a radiographic study, therefore, is not indicated.
This is a part of the December 2019 CHR VOICE.
Norbert Gleicher, MD, leads CHR’s clinical and research efforts as Medical Director and Chief Scientist. A world-renowned specialist in reproductive endocrinology, Dr. Gleicher has published hundreds of peer-reviewed papers and lectured globally while keeping an active clinical career focused on ovarian aging, immunological issues and other difficult cases of infertility.
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