First, why does she think this? She tells Jamie it's because she tried without result with Frank - but why does she think this is due to her biology and not Frank's? By the late 1940s, we in the advanced part of the world knew that infertility could be due to the male as well as the female. Did Claire go to a doctor in the future, who told her she might have difficulty conceiving? If so, why didn't she say this to Jamie, rather than the vaguer concern that she might not be able to give him children? (Again, I haven't read the novels, yet.)
Jamie's reaction is also of interest. To Claire, he's his loving and understanding self, as expected, even saying that's maybe it's for the best, since childbirth was a dangerous event for the mother in those days. But when the camera show us his face, and Claire can't see it, he is indeed at least a little disconcerted by this news.
All of which means we can expect some more important developments regarding Claire and Jamie having children at some point in the future of this narrative. Eternal optimist that I am, I'm expecting that Claire can indeed have children with Jamie, but that's based on no evidence at all.
Otherwise, the plot developed with some nice twists tonight, notably Jamie back in the hands of Black Jack and the British. Black Jack has been largely missing since the very beginning of this second half of the first season, and it's time for him to rear his head again. Whatever he may want to do with Jamie, he won't be able to resist using him to get another shot at Claire, which means there'll be some good Outlander television ahead this season.
See also Outlander 1.1-3: The Hope of Time Travel ... Outlander 1.6: Outstanding ... Outlander 1.7: Tender Intertemporal Polygamy ...Outlander 1.8: The Other Side ... Outlander 1.9: Spanking Good ... Outlander 1.10: A Glimmer of Paradox ... Outlander 1.11: Vaccination and Time Travel ... Outlander 1.12: Black Jack's Progeny
Sierra Waters series, #1, time travel