GP Rob says:
Try to avoid getting bitten in the first place. Wear pale-coloured long sleeves and trousers, and socks and shoes at dawn and dusk and when walking in long grass. Use fragrance-free toiletries and don't wear perfume, which can attract mosquitos.
It's not known whether chemical repellents, the commonest being DEET, are safe in pregnancy or not, so generally the advice is to avoid using them. If you do use them, use small amounts.
A good natural repellent is citronella. Again, its safety in pregnancy isn't known, but most people feel confident about using it. Applying it to your clothes rather than directly on your skin lessens the risk of it being absorbed into the body.