I agree with Frogspoon, and I'm amazed by how hard-hearted, mean-spirited and selfish many of the contributors to this thread are advising the OP to be.
OP's DD is 21 years old, pursuing an unglamorous and very sensible career, and studying for a degree programme which much more demanding than most others. My extremely bright DH did engineering 3 years ago and worked 14 hr days as standard after 1st year, going up to 18 hr days before exams and in the final month of each term - that's more than any of my friends who were the top medics, dentists, arts students, economics students etc in their year at the same (RG) university.
Unless OP lives in a mansion and never asks the DD to "just watch so-and-so" etc, the DD's relationship with her existing much-younger siblings will already be damaged by the fact that their noise, mess, and needs intrude on her adult life on a day-to-day basis, at a time when most of us could expect our family set-up to have moved on. And, by the sound of it, she doesn't get much that's positive out of her family relationships in order to compensate for that. TBH, in her shoes I would resent the younger siblings - and my parents for inflicting them on me.
When you become a parent, you make a commitment to your children to support them until they are able to be independent - that's very rarely as soon as they hit 18 or 21 any more, for reasons that are completely beyond their control. OP's DD, at 21, would probably much prefer that the property bubble hadn't made basic accommodation so ridiculously expensive, and that zero hours contracts weren't the norm for an estimated 2m of our workforce, and that the economy hadn't crashed out and left millions of people unemployed or underemployed. As she can't change any of that, she has taken the very sensible step of pursuing a vocational degree in a subject which the government has pinpointed as having a skills gap (whether that's strictly true or not for new grads is another matter).
It's pointless talking about how OP's DD's circumstances compare to those of other posters. Years ago, young adults may have stayed at home for longer and had far more siblings with whom to share space and resources - but they weren't generally under so much pressure to study and building up their CVs to the level that today's young people are if they want to be able to gain professional employment, so the younger siblings would have been less of an obstacle to happiness. As regards the "when I was her age I was working/living away from home" posts - well, lucky you! In the DD's position, wouldn't you want to be living elsewhere too, if it was a realistic and responsible financial decision. Since it's not, OP's DD has chosen a very sensible path towards that goal (and is working part time already), but she has to finish training before she can afford to live independently.
Until then, her parents' home is also her home and she is absolutely entitled to feel as she does about new additions to the family which detract from her quality of life, without being viewed so negatively by the OP and some of the contributors to this thread. I'm not suggesting for a second that the OP should feel bad about being pregnant - that is, after all, her choice and her husband's choice to make. However, I do find it very sad that so many people are directing so much scorn, disappointment, disapproval and so little empathy and understanding at a very young adult who (by the sound of it) many parents would be delighted to call their daughter.