Human chorionic gonadotropin (HCG) pregnancy tests have the following specific applications:

I. Early Pregnancy Diagnosis

1. Home Pregnancy Testing
   The most common application of HCG pregnancy tests is home – based pregnancy detection. When a woman suspects she is pregnant, she can use a pregnancy test stick or test strip to test her urine. These testing tools utilize the antigen – antibody reaction principle, where the antibodies can specifically recognize HCG in the urine. Generally, shortly after the fertilized egg implants (usually around 7 – 10 days), the HCG level in the pregnant woman’s urine rises to a detectable level. For example, after a woman’s menstrual period is delayed, she can obtain a preliminary pregnancy result within a few minutes through a simple urine test, providing her with a convenient and rapid self – testing method.
2. Early Diagnosis in Hospitals
   In a hospital setting, in addition to urine tests, blood tests for HCG can be used for more accurate early pregnancy diagnosis. The HCG level in the blood rises earlier than that in the urine, and blood tests can more precisely quantify the concentration of HCG. After assisted reproductive technologies such as in – vitro fertilization (IVF), doctors will determine whether the embryo has successfully implanted by regularly detecting the HCG level in the blood. For pregnant women at risk of ectopic pregnancy, blood HCG testing combined with other methods such as ultrasound examination can help doctors detect abnormal pregnancy conditions earlier.

II. Monitoring Pregnancy Progression

1. Monitoring of Normal Pregnancy
   During a normal pregnancy, the HCG level changes with the increase of gestational weeks. By regularly detecting the HCG level, doctors can understand the development of the embryo. In the early stages of pregnancy, HCG usually doubles every 48 – 72 hours. If the growth rate of HCG is too slow or it decreases instead of increasing, it may indicate poor embryo development or a risk of miscarriage. For example, when a pregnant woman shows symptoms of threatened miscarriage such as light vaginal bleeding, detecting the HCG level helps doctors determine whether the embryo is still developing normally, so as to take corresponding measures.
   In the middle and late stages of pregnancy, although the HCG level does not increase as rapidly as in the early pregnancy, it can still be used as a reference indicator. Some obstetric complications such as placental insufficiency may lead to abnormal HCG levels, and detection can assist doctors in diagnosis.
2. Detection of Multiple Pregnancies
   The HCG level is usually higher in multiple pregnancies than in singleton pregnancies. If a pregnant woman’s HCG level is significantly higher than the reference value for singleton pregnancies at the corresponding gestational week, combined with ultrasound examination, doctors can preliminarily determine whether it is a multiple pregnancy. This is of great significance for subsequent pregnancy management and delivery planning, as multiple pregnancies may bring higher pregnancy risks such as premature birth and gestational hypertension.

III. Diagnosis of Pregnancy – related Diseases

1. Diagnosis of Ectopic Pregnancy
   Ectopic pregnancy is a dangerous pregnancy condition where the fertilized egg implants outside the uterine cavity. In the early stage of ectopic pregnancy, the HCG levels in the urine and blood of pregnant women are usually lower than those in normal intrauterine pregnancies, or the HCG growth is slow. By detecting the HCG level and combining it with ultrasound examination, doctors can improve the early diagnosis rate of ectopic pregnancy. If ectopic pregnancy is suspected, doctors will closely monitor the changes in HCG. Once diagnosed, timely treatment is required to avoid serious consequences such as tubal rupture.
2. Diagnosis of Hydatidiform Mole
   Hydatidiform mole is an abnormal pregnancy condition where there is no normal embryo in the uterus, but rather it is filled with grape – like vesicular tissue. When a woman has a hydatidiform mole, the HCG level in her body will increase abnormally, much higher than the level in normal pregnancies at the same stage. Detection of HCG combined with ultrasound examination can help doctors detect hydatidiform mole in a timely manner. After diagnosis, uterine evacuation surgery and other treatments are usually required.
3. Diagnosis of Trophoblastic Diseases
   This includes invasive hydatidiform mole and choriocarcinoma. These diseases are related to pregnancy, and the HCG levels in patients will change abnormally. After treatment, by continuously monitoring the HCG level, it is possible to determine whether the disease has recurred. For example, during the chemotherapy of choriocarcinoma patients, regular detection of HCG can evaluate the treatment effect. When the HCG continuously drops to the normal range, it indicates that the treatment is effective; if the HCG rises after a decline, it may indicate a recurrence of the disease.