However many say their conditions return, or worsen, after childbirth.
Two clinical trials are being held in France and California. Researchers want to find out whether increasing levels of progesterone or oestrogen could protect women from relapses after childbirth.
The MS Society has welcomed the study but urged some caution. Chief executive Simon Gillespie said: “This is encouraging. Some of the research going on, particularly the research in France at the moment, gives a potential clue and a potential way forward on treatments but long-term use of hormones is problematic. It does cause risks.