HCG, commonly known as the “pregnancy hormone,” is the substance detected by home pregnancy tests. Its presence indicates that a fertilized egg has implanted in the uterus. Experiencing vaginal bleeding after a positive test result can be confusing. While the body can produce blood during this time, a true, normal menstrual period cannot occur when hCG is present.
Why hCG Prevents a True Period
A normal menstrual period results from a hormonal sequence that ends with the shedding of the uterine lining, the endometrium. This shedding is triggered by a drop in progesterone when pregnancy does not happen. Once implantation occurs, the embryo produces hCG, which signals the body to stop this hormonal drop.
The role of hCG is to rescue the corpus luteum, a temporary structure in the ovary that produces progesterone. HCG ensures the corpus luteum continues its function, sustaining high progesterone levels. This keeps the endometrium thick and stable, supporting the pregnancy.
High progesterone levels block the hormonal trigger for menstruation by preventing the uterine lining from breaking down. Therefore, any bleeding experienced while hCG is present is not a true period. This bleeding originates from other sources and often differs in flow and duration from a regular menstrual cycle.
Common Non-Menstrual Causes of Bleeding
Bleeding or spotting early in pregnancy is often a localized event. Implantation bleeding occurs when the fertilized egg burrows into the uterine wall. This usually results in light, pink or brown spotting, often occurring around the time a period would have been due.
Increased blood flow to the cervix is another frequent source, caused by pregnancy hormones. This heightened vascularity makes the cervix more sensitive, and light spotting can occur after sexual intercourse or a pelvic examination. Such bleeding is usually temporary, caused by simple irritation of the cervical tissue.
A subchorionic hematoma, a pool of blood near the gestational sac, is a common cause of first-trimester bleeding. The flow can range from light spotting to heavier bleeding with clots. Most hematomas resolve on their own without negative consequences for the pregnancy.
Hormonal fluctuations can cause temporary breakthrough bleeding as the placenta takes over hormone production. Bleeding may also indicate a threatened miscarriage. Any instance of bleeding should be reported to a healthcare provider for evaluation.
When Bleeding Requires Immediate Medical Attention
While light spotting is common in early pregnancy, heavy bleeding requires immediate attention. Heavy bleeding is defined as soaking through one or more sanitary pads in an hour for two or more hours. Passing large blood clots or tissue fragments also suggests a more serious event, such as a miscarriage.
Bleeding accompanied by severe abdominal pain, especially if localized to one side of the pelvis, is a warning sign. This combination can indicate an ectopic pregnancy, where the fertilized egg has implanted outside the uterus. Ectopic pregnancy can become life-threatening if the tube ruptures.
Other symptoms warranting emergency care include:
- Severe dizziness or feeling faint.
- Pain in the shoulder, which can occur if internal bleeding irritates nerves under the diaphragm.
- Any gushing, bright red flow.
- Bleeding accompanied by a fever.