It is a common misunderstanding that a person can experience a true menstrual period while pregnant. The bleeding some people report during pregnancy is not menstruation, but rather vaginal bleeding that occurs for other reasons. Because this bleeding can be alarming, especially if it appears around the time a period would normally be expected, it is important to understand the biological difference and the various causes behind the bleeding. Approximately 15% to 25% of all pregnant women experience some form of vaginal spotting or light bleeding during the first 12 weeks of pregnancy. The causes of this bleeding vary significantly depending on the stage of pregnancy, ranging from benign events to more serious conditions that require immediate medical attention.
The Biological Impossibility of a True Period
A true menstrual period is the shedding of the uterine lining, the endometrium, which occurs when a fertilized egg does not implant in the uterus. This process is controlled by the cyclical rise and fall of reproductive hormones. If a pregnancy is established, the body’s hormonal environment changes drastically to support the developing embryo.
Once a fertilized egg successfully implants, the body begins producing human chorionic gonadotropin (hCG). This hormone, along with elevated levels of progesterone, signals the ovaries to halt the menstrual cycle. Progesterone maintains the thickened, nutrient-rich uterine lining, transforming it into the decidua. Because this lining is actively preserved, the physiological process of shedding the endometrium—which defines a period—cannot happen.
Common Causes of Vaginal Bleeding in the First Trimester
Bleeding during the first trimester is the most frequent, with studies indicating that up to one-fourth of pregnant individuals experience it. One of the most common causes is implantation bleeding, which happens when the fertilized egg embeds itself into the uterine wall. This light spotting usually occurs around 10 to 14 days after conception, near the time a period would have been expected.
Changes to the cervix are another frequent source of light bleeding. The surge in pregnancy hormones causes increased blood flow to the cervix, making the tissue more sensitive. This increased vascularity can cause spotting after sexual intercourse or a pelvic examination. A subchorionic hematoma, a blood clot that forms between the uterine wall and the gestational sac, is also a common cause. In many cases, these hematomas resolve on their own, but they can be associated with a higher risk of early pregnancy loss.
Vaginal bleeding can also be a sign of complications such as a miscarriage or an ectopic pregnancy. A miscarriage is the loss of a pregnancy before 20 weeks, often beginning with light bleeding that progresses to heavier flow and cramping. An ectopic pregnancy, where the fertilized egg implants outside the uterus, is a potentially life-threatening condition that causes bleeding and pain and requires immediate medical attention.
Causes of Bleeding After the First Trimester
Bleeding in the second and third trimesters is less common than in the first, and it is generally considered a more serious sign. One potential cause is placenta previa, a condition where the placenta partially or completely covers the opening of the cervix. This often presents as painless, bright red bleeding.
Another serious cause is placental abruption, which involves the premature detachment of the placenta from the uterine wall. This detachment can interrupt the fetus’s supply of oxygen and nutrients and is often accompanied by severe abdominal or back pain. Bleeding in the later stages of pregnancy may also signal the onset of preterm labor, which occurs before 37 weeks of gestation. Signs can include light bleeding, an increase in vaginal discharge, or the passing of the mucus plug, sometimes referred to as “bloody show.”
Recognizing When to Contact a Healthcare Provider
Any instance of vaginal bleeding during pregnancy should be reported to a healthcare provider for evaluation. Certain symptoms, however, warrant immediate medical contact or a visit to the emergency room. Heavy bleeding, defined as soaking through a sanitary pad in an hour or less, requires urgent care.
Bleeding accompanied by severe abdominal pain, cramping, or backache is also a cause for immediate concern. Other danger signs include passing tissue or large clots, experiencing dizziness or fainting, or having a fever or chills. Even if the bleeding is light, note the color, consistency, and duration to provide the healthcare provider with the necessary information.