The HCG pregnancy test plays a crucial role in the diagnosis of ectopic pregnancy:

Indicating the Possibility of Abnormal Pregnancy
In a normal intrauterine pregnancy, after the fertilized egg implants, the trophoblast cells secrete a large amount of HCG, and the HCG levels in the blood and urine increase as the gestational weeks progress. In the case of ectopic pregnancy, since the embryo implants outside the uterine cavity, such as in the fallopian tube, ovary, or abdominal cavity, the blood supply and nutritional environment in these areas are not as good as those in the uterus. This restricts the growth and function of the trophoblast cells, resulting in a generally lower amount of HCG secreted compared to a normal intrauterine pregnancy. Therefore, by detecting the HCG levels in the blood or urine, if it is found that the growth rate is too slow, the level is lower than that of a normal intrauterine pregnancy at the same stage, or it decreases instead of increasing, it can initially suggest the possible existence of an abnormal pregnancy like ectopic pregnancy.
For example, in the early stages of pregnancy, the HCG level in a normal intrauterine pregnancy usually doubles every 48 – 72 hours. If consecutive detections show that the HCG does not increase according to this pattern, the possibility of ectopic pregnancy needs to be considered. This initial screening method helps doctors promptly identify potential abnormal pregnancies for further examination.

Combining with Ultrasound to Improve Accuracy
The HCG pregnancy test combined with ultrasound examination is an important means of diagnosing ectopic pregnancy. When the HCG level reaches a certain degree (usually around 1500 – 2000 IU/L), if it is a normal intrauterine pregnancy, an intrauterine gestational sac can usually be observed by ultrasound. However, if no gestational sac is detected in the uterus during the ultrasound examination while the HCG level indicates pregnancy, the likelihood of ectopic pregnancy increases significantly. Because ultrasound examination may yield false – negative results due to various factors (such as too early examination time, too small gestational sac, etc.), the detection of HCG levels can provide supplementary information to the ultrasound examination, and the combination of the two can improve the accuracy of ectopic pregnancy diagnosis.
For instance, in some women in the early stages of pregnancy, the ultrasound may not be able to detect the intrauterine gestational sac for the time being. But if at the same time, a low HCG level with slow growth is detected, doctors will highly suspect ectopic pregnancy and then arrange closer observation or further examinations.

Observing Disease Progression and Treatment Efficacy
For patients suspected of having an ectopic pregnancy, dynamic monitoring of HCG levels is of great significance. During conservative treatment (such as treatment with drugs like methotrexate), the decrease in HCG levels can reflect the effectiveness of the treatment. If the HCG level continues to decline after treatment, it indicates that the activity of the trophoblast cells is inhibited and the embryonic tissue is gradually dying, suggesting that the treatment is effective. Conversely, if the HCG level rises after a decline, it may indicate treatment failure, that the embryo is still growing, or the presence of persistent ectopic pregnancy and other conditions, requiring further evaluation and adjustment of the treatment plan.
Monitoring HCG levels is also important after surgical treatment of ectopic pregnancy. After the operation, the HCG level should gradually decline to the normal range. If the HCG level declines slowly or increases instead of decreasing, it may indicate that the surgical removal of the embryonic tissue was incomplete, or there are other remaining trophoblastic diseases. Timely treatment is needed to avoid serious complications such as persistent ectopic pregnancy and intra – abdominal bleeding.