Parenting.com mentions that there is no real cause for concern, because the muscles in the uterus and abdomen protect the child. The fetus is suspended in an amniotic fluid within the amniotic sac for further protection. The spine, pelvis and rib cage adequately shield the fetus.
About.com further notes that miscarriages from getting hit in the stomach are an old wives tale, and complications from getting hit in the stomach are unusual during the first trimester. This is because the uterus is protected by the pelvis during the first trimester, but there is a risk of injury when the uterus rises above the pelvis. Serious trauma, such as a car accident or fall, can cause damage to the placenta. Parenting.com notes that frequent cramping 12 hours after trauma to the stomach is cause for concern, along with contractions, bleeding and vaginal discharge.
Parenting.com adds that a blow to the stomach should be mentioned to a doctor. A non-stress test is used to monitor the baby's heart rate and movements, and an ultrasound pinpoints any damage or healing that took place. The mother should contact a medical professional if the child is less active in the womb. For instance, a child should move five times within two hours when the mother lies down during the second or third trimester.