My guess is that this has more to do with Mitchell than with Elizabeth -- even though when she was first kidnapped she didn't look physically mature enough to be menstruating, she did when she was found 9 months later.
Mitchell had two children in his brief first marriage that ended when he was about 21. With all the delving into his background, there hasn't been so much as a whisper of any subsequent offspring, even though we've heard plenty about his subsequent relationships and marriages through the 25+ years between the end of his first marriage and his abduction of Elizabeth, including at least one to a woman who was still of child-bearing age and whose young teenage daughter from a previous marriage he was "molesting" (presumably raping) for at least a couple of years. I think he's infertile for some reason, though I doubt he would have had a vasectomy. It's not terribly uncommon for middle aged or older men to be infertile -- it's usually something easily fixable with a minor medical procedure, but Mitchell wouldn't have been likely to pursue diagnostics and treatment.
I certainly hope Elizabeth was as "fortunate" in the STD department as she was in the pregnancy department. STDs are a major cause of infertility in men, and Mitchell seems very likely to have contracted one or more along the way, and not to have gotten prompt (or any) treatment for it.
Really, I read by 1981 he was married to his second wife, Debbie Mitchell, who had three daughters from a previous marriage. With Debbie's three children and Brian's two children, the Mitchell's had their hands full, but that did not stop the couple from having two more children soon after their marriage.
On the same day that Mitchell and Debbie divorced, Mitchell married Wanda Barzee. Barzee was a 40-year-old divorcee with six children, who she left with her ex-husband when she left the marriage. I would have thought that a lack of children with Barzee would likely have been caused by her age (over 40) and not with Mitchell who was 32 years old at the time and had already fathered four children.