It's almost 9 years ago now, but DD was a Christmas baby and I didn't buy a winter coat.
I had a loose fitting long mac type coat that I had when I needed to completely cover up. Not a pg one, just one that I already had that managed to fit over the bump until the last couple of weeks.
I generally wore plenty of layers (handy for the office and changing temps), so did buy mat vests and tops, I used to wear a shirt (either the size too large that I had shrunk from, or 1 of my 2 proper mat ones) with a maternity suit jacket over it to work. At weekends, I had a stretchy woolen poloneck from M&S (well, 2 of my 3 I sacrificed to be allowed to stretch, but they went back into shape afterwards), and the vests.
I also bought a mat long black cardi that went to my knees and tied with a string, and wore a black pashmina over that on colder days. So Christmas Day, I actually wore a vest, the black poloneck (I wear a lot of black anyway), the cardi and pashmina. A nice statement necklace. And a hat and gloves. Frosty ground, including a long walk (waddle) down the 1.5km pier near the house, standing around church yard after mass, plenty outdoors. But able to strip off the layers for indoors.
I changed into yoga trousers, the vest, and my Munster rugby top (I had bought an XXL (very much reduced price) to go over the bump and just got the sleeves shortened!!) when we got home at about 11pm that night, for the run to the hospital. I think I threw DH's down bodywarmer jacket open over the top, for that part.
It all depends on whether you will be outdoors loads and loads, what you really feel is important for a mat wardrobe and what you already have to re-purpose, and your budget. And how you feel the winter weather is going to be too.