there doesn't seem to be a lot of info about it.  i have found this:

take one arnica tablet every three hours during labour and one per day for four days after. this helps reduce bruising and soreness.  taken from: http://www.hamblys.net/shop/remedies/index.php?
cat=79&pre=1&nm=pregnancy+%26+childbirth

and this:

arnica and pregnancy

the use of arnica during pregnancy is not highly recommended. though homeopathic arnica can be beneficial for minimizing some pregnancy discomfort, large arnica doses have the ability to stimulate the uterus and spontaneously cause miscarriage or a possible premature delivery. thus, when using arnica for homeopathic reasons, it is best to consult your homeopathic practitioner and your physician. the medicinal benefits of arnica for relieving pain and trauma have been attested to by a good number of people. the side effects of arnica montana only stem from improper dosage or use, it is relatively safe to use if done so with supervision.

taken from:  http://www.nutritional-supplements-health-guide.com/
side-effects-of-arnica-montana.html

then this:

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.  taken from:http://www.babyworld.co.uk/faq/individualfaq.asp?
section=pregnancy&subsection=
medication&query=&opt=&page=&question=490

hope these help?

Hey there, I have just picked up some arnica tablets after seeing my doctor. When is the best time to start taking them? I am 38+3 saw my MW yesterday who recommened them to me. Babys head is engaged and in the right position and having twinges. Can you help please as my doctor didn't really have much to go on!! Thanks Emma xxx