Folks, if a woman shares with you that she's been praying for another child, the correct response is, "I'll pray for you, too." Please no more advice about strategically placed pillows, or tracking and charting of temperatures and discharge (gross!), or trips to the doctors to discuss special pills, or relaxing about it because she's still young. Say, "I'll pray for you, too."
Chances are, she's already feeling enough guilt over the matter already, whether because she's gained some weight, or she had to have a c-section, or she used the pill for too long, or because she thinks she's being punished by God. Whatever her reason is, don't add to her guilt by making her think she doing something else wrong. She's burdened by the Law, she's feeling the weight of sin. She's living daily with a body she feels is defective and is afraid she's the cause. Share with her the Gospel she needs to hear. Remind her of God's steadfast love, and his enduring mercy. Then do as you said you'd do, and pray.