Since most women discover that they're pregnant at about 6 weeks, and some not until three months then, taking the minimum of a month and a half, Elizabeth would be 7.5 months pregnant when Mary arrived. Three months later is 10.5 months. Taking the maximum Elizabeth would be 12 months pregnant.
Bear in mind the definition of a month in pregnancy terms. A human pregnancy is said to be 40 weeks long. If you divide that by four, you get 10 months. However, if you actually work it out based on calendar months, you find that it's nine months. So a human pregnancy is nine CALENDAR months.
Since bleeding in the early stages of pregnancy is common (25-30%) and often occurs when the woman's period is due, it isn't until the following month that a true missed period is noticed and a pregnancy test done. We know that the pregnancy is older than the bleeding suggests because we have ultrasound now, but back in Biblical times they didn't have this luxury, which goes some way to explaining the lengths of pregnancy given by the Romans you cited.
I don't think that Elizabeth having a nine month pregnancy supports in any way the Virgin Birth. Your logic is faulty.
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