answers: During pregnancy
context: Vesico-uterine pouch In human female anatomy, the vesico-uterine pouch, also known by various other names, is a second but shallower pouch formed from the peritoneum over the uterus and bladder, continued over the intestinal surface and fundus of the uterus onto its vesical surface, which it covers as far as the junction of the body and cervix uteri, and then to the bladder. This pouch is an important anatomical landmark for chronic endometriosis. Endometrial seeding in this region causes cyclical pain in women of child bearing age. This pouch is also an important factor in a retroverted uterus, which can </s> the time of ovulation—a period of high estrogen levels— the mucus is thin and serous to allow sperm to enter the uterus, and is more alkaline and hence more hospitable to sperm. It is also higher in electrolytes, which results in the "ferning" pattern that can be observed in drying mucus under low magnification; as the mucus dries, the salts crystallize, resembling the leaves of a fern. The mucus has stretchy character described as "Spinnbarkeit" most prominent around the time of ovulation. At other times in the cycle, the mucus is thick and more acidic due to the effects of </s> Endometrium The endometrium is the inner epithelial layer, along with its mucous membrane, of the mammalian uterus. It has a basal layer and a functional layer; the functional layer thickens and then is shed during menstruation in humans, as well as some other mammals including apes, Old World monkeys, some species of bat, and the elephant shrew. In most other mammals the endometrium is reabsorbed in the estrous cycle. During pregnancy, the glands and blood vessels in the endometrium further increase in size and number. Vascular spaces fuse and become interconnected, forming the placenta, which supplies oxygen and nutrition to
question: when does the uterine lining become more vascular