It is OK if you feel confident, comfortable and competent treating pregnant ladies, Jo. This applies to any client group really.
mysterious me makes some good points, and here are a few additions. See if you can read up about using reflexology during pregnancy, talk to a colleague who has experience in this area, avoid using essential oils if you are not an aromatherapist (lavender and jasmine are two I know of that are contraindicated in some stages of pregnancy), be aware of the position you are treating pregnant ladies in as leaning back can making the heavily pregnant feel dizzy and very unwell (pregnant ladies are not meant to lie on their backs for any period of time and even a Lafuma can be too much for some clients), get as much information as possible about how they are and any problems they have, what has shown up on their blood tests, and so on. Serious complications of pregnancy such as pre eclampsia and higher grade placenta praevia are contraindications - a client of mine was told that she had Grade III placenta praevia or low lying uterus, and I consulted the AoR who advised me to stop treating her.
These are just some thoughts I have on pregnant clients in general. If you find it's an area you're interested in and enjoy, and you're attracting more pregnant ladies, then a course on maternity reflexology would be an excellent thing. I got a pregnant lady as one of my first clients after qualifying, and that prompted me to go on Susanne Enzer's excellent course. I learned lots and it gave me so much confidence in my treatments and in handling this client group in general. I hope you enjoy it.