Vaginal bleeding early in pregnancy is very common and usually not dangerous. If the bleeding becomes heavy, that can be a sign of an impending miscarriage. First trimester bleeding with significant pain could be a sign of ectopic pregnancy which is a medical emergency.
It is very common to have some spotting or bleeding in the first trimester. One of the common reasons is something called implantation bleeding. This is bleeding that occurs when the fertilized egg first attaches, or implants, into the uterine wall. This often causes bleeding and is not dangerous or worrisome. This may appear as spotting or bleeding noticed when you wipe yourself. The bleeding can be anywhere from bright red, to dark, to brown, to charcoal-like. It usually goes away after a few days.
The placenta is essentially a bag of blood vessels. As the placenta develops, many new blood vessels form. These new blood vessels are fragile and can sometimes break. When this happens, bleeding behind the placenta can occur. This is called a subchorionic hematoma. The bleeding behind the placenta eventually makes its way to the cervix and out the vagina.This bleeding can be very heavy or very light. It can be bright red or dark. Sometimes there can also be clots. Usually there is an initial bleed and then it stops. This blood can take a while to completely clear out and will usually darken over time. Rarely the bleeding can cause more blood vessels to break leading to a chain reaction and cause a miscarriage. There is no way to control this. Decreasing activity or even bedrest will not change whether or not a miscarriage will happen. Increased activity can make the blood come out a bit more quickly which can be scary for people, but it will not affect the ultimate outcome. In the vast majority of women subchorionic hematoma will just causes some bleeding and nothing more.
If significant pain is present with bleeding or spotting, especially if it is on one side or the other, this could be a sign of an ectopic pregnancy. This occurs when the fertilized egg implants in the fallopian tube rather then the uterus. Thus will eventually rupture and can be life threatening. Vaginal bleeding, even if it is just spotting, in the presence of significant pain is a reason to call your doctor immediately or to go to the emergency room.
When bleeding occurs in the first trimester it is often frightening for people. There is the worry that the pregnancy will be lost. In most cases this is actually a normal part of pregnancy and all is well. I advise my patients to call if they are concerned. It is better to call for a false alarm then to have a problem and not call. Usually an ultrasound can be ordered and a heartbeat seen if the baby is big enough. Once the heartbeat is seen, the odds of a miscarriage are greatly reduced. If the baby is too small to see a heartbeat a blood test called a quantitative HCG can be ordered. This hormone goes up in a somewhat predictable fashion in a normal pregnancy. The first value may not help much but the rate of increase after 2-3 days can be very helpful to reassure you that everything is going well even very early in pregnancy.