There are a couple of info leaflets you can download here - scroll down to 'E-cigarettes in pregnancy infographic' - the one for MW and HCPs has more info.
Angharad07 you really should provide a link if you're posting things that imply there are significant risks to nicotine use in pregnancy. Many pregnant women use NRT that has been prescribed to them and many others use an ecig. While it's obviously preferable not to smoke, vape or use NRT, the very real danger of recommending only nicotine free vaping is that it won't be effective and women end up smoking again.
Also there's enough to get stressed about in pregnancy without needlessly adding to it.
NHS: You can use NRT during pregnancy if it will help you stop smoking, and you're unable to stop without it.
Tommy's: Licensed NRT products are free of charge, safe to use in pregnancy and can increase the chances of quitting successfully.
ASH (PDF link): As stopping smoking can be difficult, some women may need pharmacotherapy support. A review of the efficacy and safety of nicotine replacement therapy (NRT) in pregnancy found that NRT increased smoking cessation rates, measured in late pregnancy, by approximately 40%. The authors found no evidence that NRT used for smoking cessation in pregnancy has either positive or negative impacts on birth outcomes.
A US review of studies found that NRT use significantly decreased the risk of preterm delivery and low birth weight compared to that of smokers and also found that NRT use does not appear to increase the risk of malformations.
(The ASH factsheet is fully referenced so you can find the original studies).