Your physician's diagnosis is a logical onethat the probable cause of the discomfort you're feeling is the stretching of the round ligaments that hold your uterus in place. As your abdomen enlarges during the pregnancy, the uncomfortable sensation you feel is caused by the increasing weight of your uterus, the placenta, the amniotic fluid, and the baby. This weight pulls on those ligaments and causes an ache. The fact that you feel this sensation after sneezing or using the bathroom makes sense, as the contraction of abdominal muscles that occurs with a sneeze or cough would pull on the ligaments.
As long as the pain is occasional and not persistent there is no cause for concern. You are likely to feel these sensations more with a second baby because your previous pregnancy left your muscles weakened and less able to support your enlarging abdomen as well as before. Try doing some gentle abdominal exercises such as pelvic tilts or curl-ups to strengthen abdominal muscles. Avoid doing full sit-ups or exercises that require you to lift both legs at the same time. Also avoid lifting heavy objects. Try splinting your abdomen with your hands when you sneeze or use the bathroom, to see whether that support will decrease the ache. Getting off your feet and resting will usually bring relief.
Preterm labor contractions are another possible cause of abdominal pain, aching, or pressure sensations during pregnancy. Be sure that you discuss this with your physician so that he or she can help you identify whether you are having any other symptoms that might be related to pre-term labor. These would include lower back pain; a watery vaginal discharge, either pink or brown in color; menstrual-like cramps, with or without diarrhea; nausea or indigestion; or leaking of amniotic fluid from the vagina.
The fact that you did not mention any of these symptoms would suggest that this is not what you are experiencing, but it is a good idea to be aware. No two pregnancies are ever the same, even for the same woman, so you are very wise to be observant and to inform your physician about your symptoms.