Worming pregnant horses in the first stage of pregnancy is not safe, due to potential malformations during organogenesis. After the first 60 days, ivermectin or benzimidazole are safe to use, but always checking the manufacturers instructions.
Some ivermectin wormers brands are Virbac, Equimed. Panacur is a benzimidazole wormer.
From the Merc Veterinary Manual:
Parasite Control During Pregnancy in Horses
By Patricia L. Sertich, MS, VMD, DACT, Associate Professor of Reproduction-Clinician Educator, New Bolton Center, School of Veterinary Medicine, University of Pennsylvania
Most anthelmintics are safe for use throughout pregnancy, but precautions and contraindications on package inserts should be heeded. In general, anthelmintics should not be administered to mares during the first 60 days of gestation (organogenesis). ../.. Pregnant mares should be administered ivermectin or a benzimidazole 1–3 days before foaling to prevent lactogenic transmission of Strongyloides westeri, which can cause diarrhea in young foals.
For my purposes I'm using EQUIMEC® Plus Tape. On the product information:
IVERMECTIN 8 mg/g and PRAZIQUANTEL 100 mg/g
.../ EQUIMEC Plus Tape has a wide margin of safety at the recommended
dose level. It may be used in horses of all ages. Mares may be
treated at any stage of pregnancy. Stallions may be treated without
adversely affecting their fertility.
This is contradictory to the advice given in the Merck Manual regarding early pregnancy, my take on these types of contradictions, if in doubt, don't. Though that's a personal choice.