You don't need to eat fatty fish or take fish oil to have a healthy pregnancy, even though many health professional with say so. In fact fish contains not only mercury and other contaminants, it's not sustainable or ethical to eat fish for it's omega fat content. The reason fish contain omega-3 fatty acids is because they eat marine microaglae.
Mircoalgae contains the original source of EPA and DHA which are essential fats needed for your brain. So why would you eat fish when you can go directly to the source: algae!
I'd love to dive deep into what exactly EPA and DHA are, as I've done so much research regarding them but I'll point you to one of the best pieces of information you could quickly watch to learn more. One is by my favourite plant-based doctor Dr. Brooke Goldner - in this video she explains how your body converts ALA into EPA and DHA. If you are healthy person who doesn't eat any animal foods and eats plenty of omega 3 rich foods, your body will most likely be making DHA and EPA. However, conversion rates are hard to figure out person to person therefore taking a supplement, especially during pregnancy and for children is a very good safe guard. I don't want to mess around with brain development so I take Dr Brooke Goldners advice and supplement with 100% plant DHA and EPA - and so do both of my children.
There are plenty of vegan algae DHA/EPA products on the market. I've tried a few and finally found one I like. I used to take Deva Vegan Liquid DHA-EPA, but I must be honest, I didn't take it very often as it tasted so nasty. In the last two years I found V-Omega 3 which is sold by a New Zealand company. It's source from marine mircoalgae, free from mercury and other contaminants - it has no fishy smell or after taste either. It also has an added bonus of plant-based vitamin D3, although it's not much and I still do supplement with D3 anyways.