Can I have a baby and finish a Ph. D.?

Go for it! I'm sure you can make it work. You'll learn to be very efficient when you do things. I'm in graduate school and other students with children simply budget their time effectively. Everyone worries about money when expecting a child. You might be surprised by the amount of money you can save if you are really conscious of your spending. Also if you decide to pursue your PhD in the life sciences (biology, chemistry, etc) most schools provide a stipend ($25,000 give or take) and health insurance. I don't know if engineering is the same. Congratulations and good luck!