Hi, welcome to HCM.
It is not premature . Limbs
should be of 20 weeks length. If they are short by more than 2 weeks then its a pathological condition associated with disease. In such cases combined opinion of radiologist
and gynecologist will be very helpful to continue pregnancy
Consult your gynaecologist as soon as possible because after 20 weeks decision making is quite difficult .
Hope I have answered your query.