The real answer is;
"
Because I said, n00b.
It's jacking material, who gives a fk"
--
Author
....
The nerd answer is;
Because her 'impregnation' is in fact only a visual manifestation of her ability to act beyond her programming, I.e. to create another program/A.I. with similar capabilities as her. In other words, like Durandal (Marathon) before her, Cortana would be achieving a stage beyond sentience that simple rampancy would not properly describe. It would be more along the lines of something that goes by many different names; Enlightenment, divine ascension, axiom dharma, othli nezahualcoyotl, the path of Doljonijiarnimorinar, kamidogu, disroute bodhisatva, etc.
Durandal (Marathon) was an A.I. like Cortana. He sought to achieve godhood as a means to escape his only limitation. Cortana being able to create another being like her & Durandal, a functionally immortal, extremely powerful being, would effectively make her at least 'god-like' by virtue of the achievement. Simply put; Creating another 'Cortana' isn't about Cortana taking the e-peen or getting preggers, it's about ascending to a higher plane of existence where the 'old Cortana' would be a de-facto lesser being by definition.
- NOTE: Prior to the Halo franchise, Bungee had a franchise called 'Marathon' with very similar themes, weaponry & A.I. characters. I'm not pulling this out of my ass here. Durandal is what Cortana was directly based off of.
tl;dr
Not 'preggers'. Her appearance is simply to represent the process. She is likely 'evolving' further.... again, we have Durandal as a direct reference for this and it is in this instance that his example is applicable, I.e. Durandal going from formless A.I. that 'opens doors' to a literal god in the form of an indestructible super-vessel, process through which he changed a lot. Basically; Pregnancy = Transformation ---> ascension
Less T&A robo-waifu and more like invisible robo-spirit goddess.
.....
This comment is
vastly more masturbatory than anything anyone here will be doing with the video in question.