It isn't a true empire line dress, which is what I had been looking for, but my efforts to convert it would work for almost any pattern with a below the bust seam / empire line style.
Here, bellow, is what I mean by a true empire line. Some empire line tops or dresses already have the skirt part gathered into the below the bust line, others may be more fitted. As you can see, this is an example of a fitted style from a vintage pattern.
First, make sure the size you will use is large enough to accommodate a fuller bust! Then, it's easy enough to allow for an expanding waistline. Here are the steps.
2) The bodice parts: bust size will increase in pregnancy, but if the pattern comes in different sizes you can probably just go up a size or two. Then it shouldn't need altering, especially if you are going to use a jersey or other stretchy fabric (advised). However, if you want to allow for some comfort later on, you could perhaps add an inch or two to the centre front (thus increasing the size by 2"-4"). Recognise that this will also widen the neckline, and you will also have to adjust the front neckline facing too. Alternatively, you can add an inch either side of the neck line by splitting the front bodice pattern down from mid-shoulder to the under-bust seam and adding in a bit of extra paper. That will widen the chest measurement without affecting the neckline. If there is an under bust dart, you need to retain that. If you've added in extra on each side, you'll have to re-draw the dart. You could also increase the bust side dart a little. You probably don't need to do anything to the back bodice (especially if you are using a size or two larger pattern).
3) Now to the front skirt piece - usually this will be 'cut on the fold', so there is no centre seam in the front of the top or dress. Again, you need to lose any waistline shaping, and remove the front dart markings - you won't be using these.
Take the side lines straight down from the under-bust seam (or, if the skirt pattern already has a flare, follow that).
4) Decide whether you'd like to have a gathered or pleated skirt part in the front. If your pattern piece says "Place on the fold", it's very easy to expand it. If it's already a loose-fitting design, and especially if you are using a stretch fabric, you'll probably only need to add 10"-12" or so, so the easiest way is just to place the pattern piece 5"-6"away from the fold. (Less, if you are already using a pattern which is a larger size than normal.) If it's a closer fitting design, you may need to add much more. Typically a pregnancy belly can go up between 12" and 20" (plus) by full-term. If you want a gathered front, find the centre point and mark it (or put a pin in), then gather the top of the skirt and draw it up before attaching to the bodice part. Make sure your pin or marking is at the centre front of the bodice part. I would keep the gathers concentrated towards the middle and a bit less at the sides. If you want it pleated, measure the width you need to match the bottom edge of the front bodice, and work out how many pleats you need. I recommend you tack the pleats in place before attaching the skirt to the bodice.
5) Next, the skirt back piece. Most empire line dresses will have an opening up the back for a zip. You may need to lose any extreme waistline shaping, but if there are back darts you can keep them or lose them as you wish. Take the side lines straight down from the under-bust seam (or, if the skirt pattern already has a flare, follow that, as on the skirt front). Check that your side seams are the same length, or you'll have trouble later!
As I mentioned earlier, I'd been unable to find in my stash a full empire line pattern to hack, but this pattern below has a partial under-bust seam, and I have now adapted it twice as a maternity top. As the 'skirt part' at the front is already gathered, I just gathered it some more.
So all I did was to extend the width of that centre bit so it would be more gathered. It's quite neat, and, if made with a not too heavy material, it hangs quite slim, and only stretches out as needed.
In other words, it doesn't look too bulky. In fact my daughter was also happy to continue wearing the first one I made, last time, while she still had a bit of a baby belly afterwards.
I used two fabrics last time for this, a plain maroon jersey, and a mock camouflage pattern in shades of beige and coffee. Sadly, no photos exist of the garment itself, but these are the fabrics:
As the last use of this pattern for maternity was a success, I thought I'd use the pattern again, Here's the result.
Below, you can see the detail of the yoke. I put a little piping between the yoke and the gathers this time.
You'll note also that I narrowed the sleeves - I didn't really like the very wide sleeves of the pattern, so I took quite a lot of the width out. They are about elbow length.
One day, I'll get a photo of it being worn!