Previous DVT in pregnancy and another pregnancy?

Hi, my daughter had a DVT in her leg with her second child. It was discovered at 20 weeks. She was on heparin injections up to 12 hours before her scheduled c-section. She safely delivered her healthy son full-term. She resumed the injections 12 hours later until he was 6 weeks old. Later blood tests showed no blood abnormalities but a pre-disposition to making clots when pregnant. She now has told me she is considering having another child and has asked me my thoughts. To be honest, I am scared but I don't want this to affect her decision. Has anyone had a prior DVT in pregnancy and then safely had another child? What was the treatment? I will of course be telling her to see her doctor but I don't know if she should see him prior to conceiving or when testing positive. Any thoughts would be welcome. Thanks in advance.