Definitely keep!!!
My friend had her daughter when her son is 12-year-old. The girl is so sweet and smart and she is now 9-yr-old. I remembered that she told me the girl is a gift for her. I was so touched though I was in my 20's then and never thought about having kids. Since she has the girl, she feels much younger and so energetic. She is always my model and sometimes I wish I had kids with same shu3xiang4 and 12 years apart :)
My colleague gave us a big surprise at her 45. She was pregnant! We always thought they were not planning to have kids. The son is very healthy, and again smart. He is just a smaller size of his daddy. Very interesting to watch him with his daddy:) Later I got to know they tried a couple of years and gave it up. That was just an accident. How lucky! Just like you :)
As long as you are healthy, you are fine. It is not too late to take prenatal vitamins from today! Folic Acid plays important role in the first 3 months. If you have good diet, the baby should have got enough from you. My OB's theory is that baby knows what to get from mom. If we need to take vitamins and calcium etc, most fo time it is for mom's own good:) If you are worried, do all the screening tests and amnio exam and Level II ultrasound etc. Since you are 40, your OB will tell you all these.
Just relax. You will be fine and get ready to receive your gift with your dear hubby :)