Raspberry leaf tea is safe and meant to be drunk in the 3rd trimester. It's popular misconception that raspberry leaf tea is drunk to bring on labour, so if you've been led to believe that you're in good company! But it's not true at all.
I studied herbal remedies a long time ago. I'm no expert, but the woman running the course was. It was from her that I originally learnt that raspberry leaf tea is safe for pregnancy.
The point of drinking it is it's meant to be help ease labour. It also contains vitamins and minerals which are beneficial in pregnancy.
The following is taken from the first page I found via google which covered it, but it's fairly standard stuff, if you want a more medical source, it shouldn't be hard to find.
The benefits of raspberry leaf tea
"Raspberry leaf tea is nutrient rich and contains many of the vitamins and minerals necessary for a healthy pregnancy including vitamins A, C, E and B, magnesium, calcium and iron. Consuming raspberry leaf tea not only helps mother and baby to get all the nutrients they need but can also help to replenish a new mothers stores after the birth.
Raspberry leaf tea also contains the alkaloid 'fragine' which is said to strengthen and tone the muscles of the uterus, helping them to contract more efficiently during labour. Research has found that taking raspberry leaf during the weeks prior to delivery helps to shorten the second stage of labour by making contractions more effective . Some studies have also found that it reduces the need for an assisted delivery (i.e. an emergency cesarean or use of forceps or ventouse).
Sipping raspberry leaf tea during and after the birth is also said to help the uterus contract back down to size, reduce after birth bleeding and help initiate the let down of breastmilk.
While there hasn't been a huge amount of research into this area, the general consensus does seem to be that drinking raspberry leaf tea during the latter stages of pregnancy can help to make for a 'better' labour with few side effects. "
"Its commonly believed that drinking raspberry leaf tea or taking raspberry leaf tablets will help to induce labour and as such this supplement is taken by many a pregnant woman keen to speed up their baby's arrival into the world. Unfortunately this is a misconception; raspberry leaf tea doesn't actually help to bring on labour at all. Having said this it is thought to have many other benefits for the birthing process and has been used as a labour-aid for hundreds of years."
Link
The only think my old tutor warned against was drinking too much of it. She said you don't want to overdo it and over-strengthen the muscles and get mega-strong contractions. But you'd have to drink an awful lot to drink too much IIRC - like ridiculous amounts!
If anyone wants to get some, Baldwins is a lovely company where you can buy herbs online.
You can buy organic raspberry leaves loose which is probably the best if you can be bothered with the faff (Looking at the picture I reckon you should keep it hidden from errant teenagers if you want to hold on to it though, looks slightly like something I came across in my own teenage years! 
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Or you can get tea bags too.
Thanks for the reminder, off to order some now! (Think I'll get one of each, so can have teabags when I'm not up for the faff!)