Hi Pippa I was also still breastfeeding my son when I fell pregnant. He was a bit older than yours (34 months) so had dropped down to just the one morning feed (and the odd comfort feed here and there) so he was quite close to weaning anyway.
As I knew I was going to get ill again I went to GP asap and got a prescription for metocloprimide (as I had used it in previous pregnancy) to start taking as soon as nausea/vomiting got overwhelming. But she did say I would have to stop breastfeeding.
I dithered for a few days, but at about 7 weeks pregnant I was feeling really unwell, so decided to wean DS and take the medication. I explained to him the Mummy was poorly and needed to take medicines, but as the medicines could get into the milk he couldn't have Mummy milk any more. I didn't mention pregnancy as didn't want him associating his sibling with stopping having milk. He actually took it really well and we had no problems weaning at all (possibly because he was so close to weaning anyway?).
I did feel guilty weaning him, but not for long as he took it so well. I have also heard that pregnancy changes the taste of breast milk, so many toddlers self wean then anyway.
I am really glad I decided to wean when I did as I needed to take the medication and look after myself (and the new baby). I was struggling to keep myself hydrated so I knew I couldn't keep on hydrating another person too! By 9 weeks I was dehydrated and on a drip in hospital so would have been in no fit state to breastfeed anyway.
There is evidence to suggest that taking medication as early as possible if you have a history of HG might prevent the condition from escalating.
Looking back I know I made the right decision for me and DS, we still have plenty of cuddles, just no more milk. The guilt I felt from being too ill to look after him or even play with him in those dark days of the first trimester was far worse than the guilt I felt for weaning him a few months earlier than he would have anyway.
I hope that helps, it's a really tough decision to make, but you never know, your toddler may surprise you like mine did. Make sure you look after yourself though.
Just to let you know there is light at the end of the tunnel - I am one of the lucky ones and at 29 weeks now I have been functioning normally for some time (hence the silence on here). Just the one cyclizine a day to keep the edge off, so a million miles away from those dark days of the first trimester and early second trimester.