Here are some of the most common causes of lower abdominal pain in the second trimester.
Miscarriage
Pregnancy loss is rare in the second trimester, but it does happen. A pregnancy loss before 20 weeks, known as a miscarriage, usually causes severe abdominal cramping accompanied by vaginal bleeding. After you have entered the second trimester, the fetus is fairly large and will only leave your body after several strong, painful contractions.
Round Ligament Pain
Round ligament pain usually doesn't occur until the later half of the second trimester, but it is a possible cause of your lower abdominal pain. During the second trimester, your uterus has to stretch to make room for the baby's rapid growth. The ligaments holding the uterus may ache because of this stretching. Round ligament pain causes mild to moderate abdominal pain, usually focused on the left side of the abdomen. It is not a reason for concern, but mention it to your midwife or physician if you're worried.
Practice Contractions
Noticeable Braxton-Hicks, or practice, contractions are uncommon but not unheard-of during the second trimester. In general, these light "squeezes" in your uterus are painless, but some women experience a sensation of pressure or mild cramping in the lower abdomen. If you are experiencing painful Braxton-Hicks contractions at this stage in pregnancy, talk to your health care provider to rule out preterm labor as the underlying cause.
Preterm Labor
If you feel regular intervals of fairly intense lower abdominal pain, you may be experiencing preterm labor. This is particularly likely if you have other signs of impending labor, such as bloody show or mucusy discharge. During the second trimester, preterm labor is very serious; the baby's odds of survival outside the womb are slim or non-existent. Fortunately, very prompt help may prevent labor from progressing further. Talk to your health care provider as soon as possible if you suspect that you are experiencing preterm labor.
Something Else Entirely
Your uterus is very close to your digestive tract, so it's possible that you are mistaking digestive upset for a pregnancy-related complication. It's possible during any trimester to experience lower abdominal cramping caused by constipation, infection, food intolerance, anxiety or gas. Digestive difficulties are not uncommon during the pregnancy, so don't assume that all lower abdominal pain originates in your womb. However, talk to your health care provider if you aren't sure why you feel lower abdominal pain during the second trimester.