Chances of being approved for mortgage loan?

My husband and I applied for a mortgage loan about 3 weeks ago and were preapproved. We gave the lender all of our information and bank statements, W2's, tax returns and check stubs for the last 2 years before being pre approved.
So our lender emails me yesterday and says he needs my full 2 year work history for the last 2 years. I told him when we were pre approved that I had only been working for the last 20 months. So I told him again that I was not working for 2 1/2 years prior to getting this job because I was a stay at home mom and pregnant during part of that time also. He said that's fine because he can say it was maternity leave. Problem with that is, only about 1/3 of that time was maternity leave and for about 8 months before that I wasn't pregnant, I was just a stay at home mom at that time and for about 1 year after I had my youngest I stayed at home also.
So now im sensing this is going to be a problem and im terrified. We've already paid $350 for an inspection on the house we are buying and paid $1,500 earnest money deposit. We are about to pay $550 for the appraisal and that's a lot of money for NOTHING if this loan falls through in the end because im 4 months short on my work history! I just don't understand why if this was going to be a problem couldn't our lender figure this out beforehand(especially since we told him.) oh and this is for a USDA loan.
What are our chances do you think of being approved given these circumstances?

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