While it is possible for a woman to have a child if they have not yet reached menopause, the age of forty is a risky time to be considering children. You may also be in the early stages of menopause and not yet realize it, a doctor will be able to confirm or deny that.
I do not believe a doctor will tell you not to have a child without good reason. They will however wish to closely monitor your pregnancy for danger to you or your baby. Extra tests before hand, as well as extra tests during your pregnancy will be required. There are many risks that you will probably be lucky to avoid.
Everyone is different; you may be able to have children successfully till you're forty-five, while other women may not be able to have children after thirty-five. Your doctor will be able to access your health and determine if you are in well enough physical condition to consider having a child.
This website will help you make your decision as well.
http://www.pregnancy-info.net/pregnancy_35_40.html
