Human Chorionic Gonadotropin (HCG) is a hormone produced by the body during pregnancy. Its presence in the body is detectable by blood or urine tests very early in the pregnancy. According to the American Pregnancy Association, in about 85 per cent of normal pregnancies, HCG levels in the body will double every two or three days during the first eight to 11 weeks. Low HCG levels may be present in a normal pregnancy. However, in most cases, low levels of this hormone are often related to problems such as miscarriage or abnormalities in the pregnancy.
When a pregnancy terminates due to natural causes, it is called a spontaneous abortion, or a miscarriage. Some of the main reasons for miscarriage include chromosomal abnormalities in the foetus, advanced maternal age, problems with maternal health, maternal trauma, malnutrition and exposure to toxins or other harmful substances such as drugs, alcohol or nicotine. The most common signs of miscarriage include vaginal bleeding and pain in the abdomen and back. HCG is produced by the placental cells that provide nourishment to the developing foetus. When a miscarriage occurs, the placenta and the foetus are expelled from the body, either naturally or by medical procedures such as dilation and curettage (D&C). When the body loses the fetal tissue that produces HCG, its levels in the blood will drop.
An ectopic pregnancy is one that starts outside the uterine wall. Most ectopic pregnancies involve implantation in the Fallopian tube and are therefore also called tubal pregnancies. Ectopic pregnancies occur because of blockages in the Fallopian tube or abnormal shape of the tube. According to the U.S. National Library of Medicine, some other risk factors include smoking, pelvic inflammatory disease, and scarring causes by pelvic surgery in the past. Ectopic pregnancies cannot be carried to term, because the Fallopian tubes cannot accommodate a growing embryo. Removal of the embryo is essential to save the mother's life. HCG levels in ectopic pregnancies are generally low, because the placental tissue cannot develop properly.
A blighted ovum or anembryonic pregnancy is a pregnancy in which the embryo does not develop. Fertilisation occurs, with the placenta and the pregnancy sac developing normally for some time. However, when the embryo does not develop, the body miscarries the pregnancy and HCG levels will drop. Most blighted ovum pregnancies occur because of chromosomal abnormalities, improper cell division or problems with the egg or sperm. In some cases, the body may naturally expel the uterine lining, but sometimes a diagnosis is only made after an ultrasound.