Menstrual cycle

The menstrual cycle, also known as a period or that "time of the month", was experienced by female humans. (PROSE: The Adventuress of Henrietta Street) In the hermaphroditic Chelonians, it was known as the Time of Blood. (PROSE: The Highest Science)

In humans
Peril Bellamy once noted that she became "emotionally labile" just before her period. Lorris Hunsdon was slightly bothered by Peril talking about her menstrual cycle. (PROSE: Bernice Summerfield and the Library of Books)

Some traditions placed great importance on the menstrual cycle. Witch-cults would not admit a young woman until she had begun to menstruate. (PROSE: The Adventuress of Henrietta Street)

When Captain Geoff Paynter accused Tegan Jovanka of being on her time of the month, she called him a "complete and total dickhead". (PROSE: The King of Terror)

The Thirteenth Doctor took Yasmin Khan to the TARDIS Hungarian bathroom to make her feel better during her time of the month, giving her a hot water bottle (to sooth Yaz's abdominal pain), a bar of chocolate, and telling her the story of how her eleventh incarnation met Amelia Earhart. (PROSE: Chasing the Dawn) Chocolate was known to force the production of serotonin, giving it a reputation for both calming and "synchronising" a woman's menstrual cycle. (PROSE: The Adventuress of Henrietta Street)

When a Strange Alien Device turned Ianto Jones into a woman, Jack accused him of being moody and wondered if he was at that time of the month. Ianto was slightly horrified by the idea and did not know how to tell if that was the case. (PROSE: Almost Perfect)

When women were being driven insane by their heightened and scrambled senses, Henry "Cash" Collins made a joke about a wife going after her husband with a cleaver while on her time of the month. (PROSE: Exodus Code)

Chris Cwej received icy stares from Roz Forrester when it was that time of the month. (PROSE: Lungbarrow)

In the far future, the Braxiatel Collection created a copy of Bernice Summerfield. On days 256 and 1024, she noted in her diary that she did not menstruate. (PROSE: A Bell Ringing in an Empty Sky)

In other species
Twenty days into the occupation of Vaagon, Chelonian General Fakrid did not receive a scheduled dose of fertizol to stimulate his brood cycle. This caused him to become unhinged as he entered the "Time of Blood". Lower ranked Chelonian soldiers had their brood cycle deactivated. (PROSE: The Highest Science)