A chemical pregnancy is a pregnancy in which the woman tests positive for pregnancy but miscarries before anything can be seen in the uterus on ultrasound.

This is also known as a very early miscarriage. In these cases, an egg was fertilized, implanted in the uterus, but then could not continue to grow for some reason.

Unlike other miscarriages, which typically occur before the 20th week of gestation, chemical pregnancies occur a week or two after implantation.

Experts are unsure what causes a chemical pregnancy, but most believe it happens for the same reasons other miscarriages happen — abnormal chromosomes in the developing embryo.

Abnormal chromosomes can be the result of many factors, such as poor quality of the sperm or egg, genetic abnormalities passed down by the mother or father, or an abnormal cell division of the fetus.

Experts believe half of all chemical pregnancies are due to some form of chromosomal abnormality. Additional potential causes may include:

Infections such as toxoplasmosis, chlamydia, or syphilis

Systemic illnesses such as untreated thyroid disease

Uterine abnormalities (congenital and acquired)

Abnormal hormone levels

Luteal phase defect

Inadequate uterine lining

Implantation outside the uterus

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Chemical pregnancy