Yeah, just read the instructions. If it says it's safe for pregnant mares use it. If it says it's not or doesn't say anything about that, don't use it. I only use fenbendazole (Safeguard) and ivermectin on pregnant mares.
Edited to add that, apparently, it's not safe for pregnant mares and foals younger than 5 mos. of age:
Animal Safety:
Zimectrin Gold (ivermectin/praziquantel) Paste may be used in horses five months of age or older. Zimectrin Gold Paste has not been tested in foals younger than five months of age, in pregnant mares or breeding stallions.