Both me and a friend had this "diagnosis" of a low-lying placenta during our first pregnancies. We were pregnant at the same time, and had ultrasounds in the second trimester both of us were really excited about the possibility of finding out the gender of our babies at that scan. Within weeks of each other, my friend was first told that she had a "low-lying placenta, which will probably mean a c-section for you". At that point, the placenta was not even covering the cervix a condition which is known as placenta previa and which means that the placenta is physically blocking the birth canal, meaning the baby cannot pass through.
When I went for my ultrasound a few weeks later, I was also told I had a low-lying placenta. All scared, I asked, "Does that mean I need a c-section?" because that is what my friend was told. My gynecologist started laughing and asked if I were kidding. Apparently, a low-lying placenta is not at all an uncommon occurrence during the second trimester. It does not mean that the placenta will go on to become a complete previa. Quite the opposite, in fact. If a placenta covers the cervix during the second trimester, it is less likely to "move out the way" later on, but it can happen. As the uterus grows, the location of the placenta will be higher in comparison to the cervix. If your placenta is located relatively close to the cervix, it will most likely not be low-lying any more. Don't start worrying about a low-lying placenta until well into the third trimester of your pregnancy.