Yes, she can unless the officer at the border believes she will become a public charge. How does she intend to pay the $10,000-20,000 for an uncomplicated delivery if she has no health insurance? Unless she has health insurance to cover her, it would be in her best interest to deliver at home and come with an infant.
Note, an anchor baby provides no benefit to a parent until age 21. If she is entering in late pregnancy with the hope that a US citizen child will make it easy for her and her J-1 sponsor to remain in the US, she is mistaken about the effect of a US citizen child on a family based immigration.