Is there one? Because this question can pretty obviously be answered by the two Wikipedia pages.
Or is it necessary to use the word womb or uterus?
I don't believe the Wikipedia page really answers this question. Concerning the stomach, Wikipedia says:
The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital digestive organ. In the digestive system the stomach is involved in the second phase of digestion, following mastication (chewing).
In humans and many other animals, the stomach is located between the oesophagus and the small intestine. It secretes digestive enzymes and gastric acid to aid in food digestion. The pyloric sphincter controls the passage of partially digested food (chyme) from the stomach into the duodenum where peristalsis takes over to move this through the rest of the intestines.
The stomach page also disambiguates to abdomen, but I don't think that's much more helpful:
The abdomen (less formally called the belly, stomach, tummy or midriff) constitutes the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the frontal part of the abdominal segment of the trunk, the dorsal part of this segment being the back of the abdomen. The region occupied by the abdomen is termed the abdominal cavity. In arthropods it is the posterior tagma of the body; it follows the thorax or cephalothorax. The abdomen stretches from the thorax at the thoracic diaphragm to the pelvis at the pelvic brim. The pelvic brim stretches from the lumbosacral joint to the pubic symphysis and is the edge of the pelvic inlet. The space above this inlet and under the thoracic diaphragm is termed the abdominal cavity. The boundary of the abdominal cavity is the abdominal wall in the front and the peritoneal surface at the rear.
I don't think either of those pages will be of much help to an English novice who is trying to learn if, when a woman is pregnant, it's okay to say: She has a baby in her stomach.
I've seen a lot of questions on ELL that were general reference, and I've even voted to close some of them, but I don't think this one is a very good example. This question touches on many subtle nuances of politeness, etiquette, vernacular, and context. A doctor might talk about the baby in the uterus, but the baby's mother is more likely to use tummy when talking to the 3-year-old, soon-to-be big sister. I think it's a great ELL question, in that it would be hard to get the skinny on proper usage from dictionaries and encyclopedias alone.