I used to work in newborn nursery. You cannot tell at 5 months whether a woman had prenatal care, unless you have access to medical records. (And we would not have access to those unless we knew who she was.
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They just noted that her fetus was healthy. Prenatal care is not required for a healthy fetus.
The odds are: She knew she was pregnant.
She appeared to be healthy. She probably had an unremarkable pregnancy and was fine wearing high heels. She was "youthful". (IOW: Her pregnancy at 4.5 months did not seem to be a "burden" like it is for us "old folks" or young folks with pre-existing health conditions)
Height: 5'1" to 5'3"
Weight: 110 to 115 lbs.
If her pre-pregnancy weight was 100 -105 pounds, this would be slightly less than average weight gain for 4.5 month pregnancy.
Most weight is gained at the end.
I was 4'11 and 92 pounds at my first prenatal visit with my first child. I weighed almost 130 at the end. I eventually got down to 105. I weight about 125 pounds at the end of my second pregnancy.
She still had quite a ways to go.
The odds are: She knew she was pregnant. Most women do!! She had a pregnancy that wasn't causing her to gain a bunch of weight and still allowed her to wear what appear to be 3 inch heels. I don't believe her red corduroy plants were maternity. Her weight gain was somewhat less than average, but I don't read too much into it. Lower weight gain is often a youthful trait, but that does not mean that she was "underage".