The nurse is caring for a woman with phenylketonuria who wants to start a family. Which of the following guidelines should the nurse provide the woman ？（）
A．Follow a low-phenylalanine diet before trying to conceive.
B．A low-phenylalanine diet is necessary only during the first trimester.
C．Begin a low-phenylalanine diet when pregnancy is confirmed.
D．Dietary restrictions won't be necessary.