Hi Bethan, congratulations on your pregnancy!
This is not really my area of expertise because I’ve never had a problem not getting enough calories! But here are some thoughts anyway. I’m guessing that you’re eating healthy meals, so you probably just need to fill in the gaps with some snacks. How about trying:
smoothies (my favorite is a cup of chocolate soymilk blended with a ripe banana – that alone will run you about 300 calories depending on the size of the banana – or 1 cup OJ, 6 oz vanilla soy yogurt, ½ cup blueberries and 1 cup sliced peaches – you can use frozen fruit – blended will give you one huge serving of about 360 calories)
hummus and pita bread
black bean dip and baked tortilla chips (cut a tortilla into 8 triangles and bake at 425 F for about 5 minutes, until crispy and lightly browned)
healthy granola bars (I have a recipe I could send you)
bagel with peanut (or other nut) butter
dairy-free and egg-free frozen waffles (there are some great blueberry and flax varieties)
a couple of handfuls of nuts and seeds
rice pudding, or a chocolate (or strawberry) tofu pudding
Baked Banana Dream (the recipe is on www.vegweb.com
I could go on and on, but I’m not even sure I’m on topic, so I’ll leave it at that!