Sometimes, blood clots form within the layers of the placenta. But more often than not, they heal themselves.

What it is: Also called subchorionic hematoma, subchorionic bleeding is the accumulation of blood within the folds of the chorion (the outer fetal membrane, next to the placenta) or within the layers of the placenta itself. These bleeds, or clots, can cause the placenta to separate from the uterine wall if they get too large, if they develop in a bad spot, or if they aren’t eventually reabsorbed.
 
How common is it? A good 20 percent of pregnant women will experience some kind of bleeding early in pregnancy, though it’s often hard to tell what’s causing the problem. Subchorionic hematomas are even harder to pick up because they don’t always result in noticeable spotting or bleeding, especially when they’re small.
 
Who is most at risk? There don’t seem to be any specific risk factors for developing a subchorionic hematoma in the first place, but if you do wind up with one, there are factors that can make you more — or less — likely to have a positive outcome.
 
What are the symptoms? Spotting or bleeding may be a sign, often beginning in the first trimester. But many subchorionic bleeds are detected during a routine ultrasound, without there being any noticeable signs or symptoms. 
 
Should you be concerned? You wouldn’t be normal if you didn’t worry when you see blood, no matter when it occurs in your pregnancy. And that’s actually a good thing, especially if it prompts you to get in touch with your practitioner, who can make sure there’s nothing amiss. While most subchorionic hematomas dissolve on their own, it is possible for the clot to get in between the placenta and the uterine wall, resulting in miscarriage.
 
Here’s the encouraging news: More than half of women who bleed during their first trimester go on to have perfectly healthy pregnancies. But because subchorionic hematomas have been linked to increased risk of placental abruption and preterm labor, you don’t want to ignore signs of spotting or bleeding.
 
What you should do: Call your practitioner; an ultrasound may be ordered to see whether there is indeed a hematoma, how large it is, and where it’s located. Depending on the findings, as well as on your practitioner’s preferences, he or she may put you on strict bed rest, insist you refrain from lifting heavy objects, and avoid exercise. In most cases, you’ll be asked to avoid sexual intercourse until the hematoma dissolves and disappears.