i didn't know what this was either, so googled it and found this on one website... it suggests that maybe safer to take after labour, not before???
Arnica is considered beneficial for healing, and is commonly taken after birth to speed up the process of recovery. Arnica products are safe to take after delivery and while breastfeeding. There is some concern about the use of arnica during pregnancy as it may stimulate the uterus and could cause miscarriage or premature delivery. However, homeopathic arnica is very much more dilute than either the herbal product or creams, and, as with most other homeopathic remedies, is considered safe to use in pregnancy. It is always wise when using homeopathic remedies to seek the advice of a registered homeopathic practitioner. Your homeopath would be the best person to advise you about the appropriate dose and the best time to start taking this remedy