Hello,
Welcome to ehealthforum and thanks for writing
The nutritional value of frozen veggies is almost same as compared to the fresh ones and there is no harm in taking them during pregnancy unless you are allergic to some particular food product.Though some nutrients as folate and Vitamin C can destroy during the freezing process but beta carotene is better preserved in frozen ones.
Rather than the type of veggies method of cooking is more important as if they are steam cooked or microwaved the nutritional value is more as compared to boiling the veggies in large amount of water.
Some tips for you:
1)Do not use frozen veggies which are more than 3 months old.
2)Cook the veggies for short amount of time.
3)It is better to steam them in little water.
4)Prefer using utensils made of glass/enamel/steel and not plastic containers.
Wishing you healthy pregnancy,
Take care!