I wouldn't tell during the process. Maybe on the point of offer. At 15 weeks I just looked podgy rather than pg anyway. Whilst I understand Dave's view, if they offer they'll have a good sense from the selection process of likely candidates for cover & you'll have 4 months or so to embed in role before ML. Taking on a new role can be stressful (and there's an extra incentive to make a clean switch) but it also means you can be open in all your planning & first actions of where you are next.
A big challenge for professional women after taking ML is resuming their climb on the ladder or maintaining their status on their return to the workplace. Getting as high as possible before you go helps this.
In theory it is illegal to sideline you because you are pregnant. In practice however....... And it would be something that was very difficult to prove too. So I guess it depends on how much you trust your boss. I think a lot of them would not be able to ignore the fact that you would soon be on Mat leave. I do also understand Dave's point of view too, and I am not sure quite what I would do.
IMHO it might be a big mistake to say nothing. In your boss's position I'd feel deceived if I eventually found out and I reckon a pregnancy, time off to have the baby, and mysterious extra child would be tricky to keep quiet either in the long or short term, especially if you try and use a rather weak 'lie'. It would be much wiser to let him know. The choice is between appearing full of integrity and respect (for your boss and colleagues) or as calculating and dishonest. The latter could have an impact on your current job which you say would remain.
I have a senior role in a large Financial Services company. I am 7 weeks pregnant with DC2. We are about to undergo a restructure at work. My boss has asked me to apply for a promotion in the restructure which I'm delighted about though there is no guarantee I would get the role (I suspect I have a 70% chance). The promotion/interview process will take place in late January when (if all goes to plan) I'll be 15 weeks pregnant. My quandary is whether to tell my (male) boss before the interview process or keep quiet and attribute my expanding waistline to Christmas excess.
I consider integrity to be incredibly important and would want my team to tell me in a similar scenario but I genuinely believe if I declare my pregnancy I'll be sidelined. I've worked damn hard to "qualify" for the promotion so feel I owe it my best shot. My current job would remain as part of the restructure so in that respect I have nothing to lose. With my DD I took six months'maternity leave and plan to do the same again.