This is bad advice particularly in relation to a new build. You will not be negatively impacted once you provide a letter from her employer stating her return date to the same position and salary. Not exactly the same but we switched our mortgage while my spouse was on the unpaid part of her maternity leave with no problems at all. We were however comfortably inside LTI/LTV limits
What might be a problem is if you have already maxed out at 3.5/4x loan to income rules for your AIP. A dependent has to be factored into your affordability. This really depends on your lifestyle and whether your current rent/mortgage plus savings can easily cover the costs associated with childcare.
3/4 months...possibly 6-8 months. Builder's schedules generally get longer. Your existing AIP could expire and you will need a new or extended AIP. You will have to provide up to date financials at some point either for AIP or drawdown. You really don't want to be in a position where you have signed a contract and you no longer have AIP. There will be penalties for not fulfilling your contractual obligations
With all of the above, you know that the baby will arrive before drawdown and you won't be able to provide the same financial information putting your AIP at risk. You really need to be proactive about this and reach out to the lender, get the return to work letter from your spouse's employer and get the AIP extended to 6 months from today.
Also, it would help if you provided some of the relevant info in terms of salaries, property price, LTV/LTI, mortgage term, ages(approx) etc. Without that detail it is hard to give a good steer on the likely outcome