It should be.
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2018
The odds of a woman having twins on new years even where on baby is born in the final minutes of the year and the other born in the beginning minutes of the next year is considered a rarity and only a handful of cases are reported each year.
No.
3rd or 4th grade, unless there are different circumstances such as the child having been held back a year (in which case they could still be in 2nd grade when they turn 9) or the child being a year ahead in school or having a late birthday right before the cut-off date (in which case they would be in 5th grade).
I hate to apply numbers so generically, but there shouldn't be any concern if she is between ~110 and ~145. At which point see what a doctor would think.