Sorry, beansprout is right, preganncy and oils are confusing, even for the qualified therapist, so even I can't be much help.
I am a qualified aromatherapist and you will see I have said this on mumsnet several times before. There is so much conflicting advice, about which oils are safe and which aren't.
Some aromatherapists will say certain oils are safe, some will say only use them at the end, some will say the same oils should be avoided completely.
Some say peppermint is contra-indicated, others say use it in the second half, the same with lavender, even though lavender and chamomile are both very gentle and can be used on babies, they are both emmanogogues - which means they promote menstruation.
Juniper is definitely to be avoided, as are cedarwood and basil. Rosemary, clary sage, jasmine, fennel, marjoram, are all oils that are generally considered contra-indicated, but many people use them at the end, and clary sage and jasmine are often used in labour.
One thing that is definite though is that all essential oils should be avoided in the first trimester. That is what they have been teaching on most courses for the last few years.
Saying that, I fell pregnant while working as an aromatherapist and used every single oil very frequently and my baby was absolutely fine.