Pregnancy

Pregnancy was the carrying of one or more offspring inside a womb, characteristically by the females of a species.

Time Lord
According to one legend, all pregnancies on Gallifrey ended in miscarriage when the Pythia cursed her world with sterility. Even Rassilon's wife was said to have endured this curse. (PROSE: Cat's Cradle: Time's Crucible) This required Time Lords to adopt the asexual "Looming" process of artificial genetic reproduction. This resulted in the elimination of direct, lineal familial relations, rendering all relationships collateral. In other words, Time Lords of the same family (or "Chapter House") could at most be only cousins of each other. However, the Pythia's curse did not seem to extend to off-worlders, as Leela was pregnant with Andred's baby. (PROSE: Lungbarrow)

According to most accounts, however, Time Lords had at least approximately human sexual relations, rendering the curse of Pythia to be at best a myth. Susan, for instance, had an apparently Earth-normal pregnancy to produce her son; the extreme xenophobia present on Earth in the 2160s and 2170s would have made her a social outcast had her pregnancy been viewed as abnormal. (AUDIO: An Earthly Child) The Master once reminisced with the Tenth Doctor about "his father's estate", which immediately discounted the idea of true sterility. (TV: The End of Time) Likewise, the accuracy of the "Pythia's Curse myth" is directly challenged by the Doctor's mention of a father (TV: Doctor Who) a brother (TV: Smith and Jones) and children (TV: Fear Her, The Doctor's Daughter) — not to mention the fact that Susan and Alex Campbell were unambiguously identified by the Eighth Doctor as his biological grand-daughter and great-grandson. (AUDIO: An Earthly Child, Relative Dimensions)

Because, however, Time Lords could regenerate into a different gender, (TV: The Doctor's Wife) reproductive gender assignments were unclear. It was unknown whether some Time Lords were predominantly male — and therefore not capable of reproduction when in a female form — or whether it was possible for all Time Lords to become pregnant while in a female incarnation.

Human
Under normal circumstances, pregnancies amongst the human and near-human were always endured by the female of the species.

The Doctor's companions
Only a few of the Doctor's companions and close allies were known to be pregnant. Most were pregnant long after they stopped travelling in the TARDIS. Dodo Chaplet, for instance, was pregnant with James Stevens' baby when she was murdered by Francis Cleary. (PROSE: Who Killed Kennedy) In 2600, Bernice Summerfield became pregnant with Adrian Wall's baby when Avril Fenman temporarily took over her body. (PROSE: The Squire's Crystal, The Infernal Nexus, The Glass Prison)

One pregnancy in a companion's family can be directly attributed to the Doctor's matchmaking. After the Tenth Doctor's repeated insistence that it would be a "good match", an alternate Peter Tyler pursued a relationship with Jackie Tyler. She was pregnant with his son by the time that the Tenth Doctor summoned Rose Tyler to Bad Wolf Bay. (TV: Doomsday)

One companion was known to be pregnant while an active TARDIS traveller. After being exposed to psychic pollen, Amy Pond dreamed that she was pregnant with Rory's baby. (TV: Amy's Choice) She later fell pregnant and gave birth to River Song, conceiving her while in the Doctor's TARDIS and it was confirmed that being conceived in the Time Vortex had given River Time Lord-like characteristics. (TV: A Good Man Goes to War)

Another companion, the male Chris Cwej, was known to have impregnated a couple of women — Ishtar Hutchings and Dep — that he met while the Seventh Doctor's companion. (PROSE: Happy Endings, The Also People)

Captain Jack Harkness, a male, had fallen pregnant at an unspecified point in his life. (TV: Everything Changes)

Other humans

 * Queen Mary I believed that she was pregnant, but this turned out to be a false pregnancy. (AUDIO: The Marian Conspiracy)
 * James Stevens's wife Natasha was pregnant with his baby when their marriage ended. (PROSE: Who Killed Kennedy)
 * Sarah Clark was pregnant with Stuart Hoskins' baby on the day of their wedding in 1987. (TV: Father's Day)
 * In 1997, Tilusha Meswani became pregnant with Phil Tarrant's baby, which became possessed by Kelzen. (PROSE: Infinite Requiem)
 * Penelope Silver became pregnant with Norton Silver's baby. (PROSE: Option Lock)
 * In 2006, Cathy Salt was pregnant when she interviewed Margaret Blaine. Blaine, who was actually Blon Fel-Fotch Passameer-Day Slitheen in disguise, planned to kill Cathy, but let her go when she learned of her condition. (TV: Boom Town)
 * Paul Langer told his son Clyde that he had impregnated Clyde's aunt, Melba. (TV: The Mark of the Berserker)
 * Donna Noble mentioned that her friend Nerys had gotten herself pregnant with twins using a turkey baster. (TV: The Doctor's Daughter)
 * In 2009, Gwen Cooper appeared to be pregnant when a Nostrovite transferred one of its eggs into her body. (TV: Something Borrowed)
 * A year later, Gwen became pregnant for real with her husband Rhys Williams' baby. This happened despite her use of contraceptive pills. Gwen blamed the various medications she had been given as a member of Torchwood Three for the failure of her contraception. (TV: Children of Earth: Day One, Children of Earth: Day Two)
 * Tameka Vito became pregnant with Scott's baby. (PROSE: Beyond the Sun)
 * In 5,000,000,053, Cheen was pregnant with Milo's baby. (TV: Gridlock)

Other species
Shub-Niggurath was pregnant when she arrived into the beginning of the universe from the previous universe. She delivered her child on Polymos, which became the Nestene Consciousness. (PROSE: Synthespians™)

In the year 200,000, the Face of Boe fell pregnant, expecting 'Boemina'. (TV: The Long Game)

Behind the scenes
A few actors made public the fact that they were pregnant while filming Doctor Who. Caroline John was probably the first, though she didn't let producer Barry Letts know about it. Nevertheless she definitely was pregnant during the filming of The Ambassadors of Death, and even performed some ill-advised stunts. (DCOM: The Ambassadors of Death) Wendy Padbury was also pregnant during the filming of The Five Doctors and had to have rather oddly-designed clothes to conceal the fact. Unfortunately, she miscarried soon after recording the story. (DOC: MM VHS 7)

At least one false pregnancy was circulated by the Doctor Who production office. Mary Tamm claimed that John Nathan-Turner invented the rumour that she left the programme was because she was pregnant. In fact, though, her daughter's birthday was proof she couldn't have been pregnant while filming even the later part of her tenure.