For the use of toning the uterus (also known to help with post-partum bleeding and increased milk supply), anything from 32 weeks is fine. If you are that unsure, ask a naturopath, but the advice they give usually varies.

I found this on the Birthrites website:

Raspberry Leaf in Pregnancy.

The study will be published this year in the 'Journal of Midwifery and Women's Health', April edition, 46(2).

A randomised, placebo-controlled, double-blind study performed by Myra Parsons and Michele Simpson in 1999-2000 at Westmead Hospital in Sydney demonstrated the safety of raspberry leaf tablets (2.4 gms daily) taken from 32 weeks pregnancy until commencement of labour. There were no side effects identified, for mother or baby, by the intake of this herb.

The analysis suggested that raspberry leaf tablets shortened the length of second stage of labour by an average of 10 minutes but made no difference to the length of first stage. It also reduced the incidence of 'artificial rupture of membrane' and forceps/ventouse deliveries. Although not statistically significant, these results are clinically significant. At the present time there is no known research on the safety of taking Raspberry Leaf earlier in pregnancy.