During the time of corsets and tight waisted dresses for women what would they wear during pregnancy?

Many thanks to /u/searocksandtrees for linking my past answer! I wrote it a long time ago and my contributions to that thread are largely frantic attempts to stem misinformation, so I will write you a fresh one as well.

The point of boned foundation garments (during the period in which they were worn) was, by and large, to give the "proper" shape to the body and to provide support. By the latter, I largely mean bust support, although advertisements and treatises promoting corsets talked up the garment's ability to support the body in general; children were also dressed in stays and corsets because they were believed to help young bodies to grow straight and strong, similar to the way that one braces a sapling. The former can be difficult to understand from a modern viewpoint - we often tend to phrase it as "the fashionable shape", which implies that women who didn't care about projecting an image of being on the bleeding edge of fashion might have forgone it, but in fact it was more a question of propriety. To not wear a corset by the mid-18th century, particularly in an English context, was to put yourself outside of respectable society; rural, working-class populations on the continent at this point were often still wearing a form of folk dress, which did often involve an unboned but still supportive garment. With the coming of the Industrial Revolution, corsetry gradually became available to all women at a wide range of price points, cementing the corset's position as a required part of the wardrobe: the soft, squishy (scientific term) look of a body without a corset would be immediately obvious to any viewer, and would represent a desire to look overly sexual in public. As a result, dropping corsetry when pregnant would be unthinkable.

The earliest evidence we have of corsets built specifically for pregnancy is in Diderot's Encyclopedia, published in 1771, in a plate showing one half of a set of maternity or gestational stays from the side. These are in almost every aspect like ordinary stays, but the center front seam is cut to form a slightly convex curve to accommodate the stomach, and the side features a slit from the bottom almost to the top, with eyelets on either side for lacing to allow the gap to be held in or let out as the size and shape of the individual pregnancy required. The clothing worn on top of maternity stays was similarly flexible: gowns were typically worn open in front over a stomacher, and the opening could simply be increased, the ordinary stomacher perhaps replaced with a larger one or covered with a handkerchief or mantelet (as shown in the 1774 satire, The Man of Business), the petticoat and apron tied looser and higher as needed. Once the pregnancy had progressed to the point that even maternity stays could not be worn at all, most women would probably stick to a petticoat and bedgown, an unfitted jacket that could be held closed with pins and one's apron, and would stay out of the public eye as much as possible. (In the case of poorer women who couldn't buy maternity stays, borrow them, or alter their own stays to have this side lacing, this stage would likely come much earlier. It's possible that this is why an apron over a baby bump was a near universal sign of pregnancy in popular satires of the day, even when the pregnant woman in the image is wearing a normal gown.)

Similar methods were used on nineteenth and early twentieth century maternity corsets - that is, side lacing. With the addition of "cups" for the breasts in corsets - not an element of eighteenth century stays - lacing was also often added there to deal with changes in the upper half of the body. The purpose of the maternity corset in this period was to support the belly from underneath, and to support the weight of one's clothing, as much as to give the proper shape. However, nineteenth century clothing was much less adjustable than that of the eighteenth century: rather than featuring lots of ties and pins, after about 1820 gowns generally fastened with hooks and eyes or buttons, meaning that any changes would have had to be made by unpicking and resewing darts and seams. The diary of Jeanette Marshall Seaton, who married in 1892 and very soon fell pregnant, gives us a very good look at what a professional-class/upper-middle-class woman would expect from her wardrobe: her high-end day and dinner dresses were slightly altered and artfully covered with a lace shawl or a scarf in order to work until midway through her pregnancy, and a traveling dress was let out extensively for the last few weeks and then put back to its original shape afterward. Two dresses were purchased/made specifically for her pregnant shape and one of those was altered afterward to fit her normal form. Between clothing being altered after maternity in order to continue to be usable, loose "undress" clothing being worn both in and out of pregnancy, and maternity clothes being handed down until worn out, we have little physical evidence of specifically maternity clothes.

There's a misconception that expectant Victorian mothers hid themselves away as soon as they were visibly pregnant, so maternity clothes didn't come into the picture - this is a misunderstanding of the term "confinement", which referred to the short period before the birth as well as the time afterward when the new mother stayed in bed or at least her bedroom (highly variable - affluent women could rest until fully healed, while working women would often be expected to be on their feet the next day) but was also a euphemism for "labor". While pregnancy was a taboo topic in open, public conversation and in fiction and fashion magazines, women were allowed to exist outside of the home while pregnant. At the same time, the pregnant figure wasn't considered a beautiful figure, and women did show concern about camouflaging it for aesthetic reasons. Throughout Jeanette Seaton's pregnancy, she prided herself on concealing her condition, and proudly noted in her diary that one female acquaintance was shocked when the birth announcement turned up in the newspaper due to her success.

For more detailed information about Jeanette Seaton and her maternity clothes, you should look up the article "The Expectant Victorian (Late 19th Century Maternity Clothes)" by Zuzanna Shonfield in the journal Costume, volume 6.