Antrum

1) In or near the stomach : 
There are two different antra that exist in or near the stomach, for example. The first is the antrum cardiacum. This is the dilation, or enlarging, that happens low in the esophagus, close to where it flows into the stomach.
The second is the pyloric antrum. It¡¯s also commonly known as the gastric antrum. This is the wider part of the pylorus, which is the narrower part of the stomach. It resides upstream from the pyloric canal and its junction of the pyloric sphincter to the duodenum, or first part of the small intestine. The gastric antrum contains mucus-secreting cells and gastrin-secreting endocrine cells.

2) In the temporal bone of the skull
The mastoid antrum can be found in the temporal bone near the middle ear. This air-filled antrum is next to mastoid air cells. It also communicates with the middle ear. The mastoid cells are thought to affect the function of the inner and middle ear.

3) In the ovaries
Follicular antra are found in the ovaries. Each antral follicle contains a fluid-filled antrum and an immature egg cell. This follicle matures when it¡¯s preparing for ovulation.
The presence of follicular antra can be analyzed as an indication of a female¡¯s ability to get pregnant. During fertility testing, doctors may order antra follicular testing. The higher the number of follicular antra, the greater probability of pregnancy.

4) In the sinuses
The maxillary antrum is the largest of the paranasal sinuses. It¡¯s commonly called the maxillary sinus, and is also known as the antrum of Highmore. It¡¯s found within the maxillary bone next to the nose, above the teeth, and under the eyes. The maxillary sinuses open into the middle meatus of the nose.

Reference: https://www.healthline.com/health/antrum