hello.. 
there are two chances i.e young fetus- might be lagging gestational age because of IUGR or else you might have got pregnant after conceiving elder one.. these can be well explained by your doctor.. needs other clarifications too to conclude anything about 2nd fetus..

@616d5a27c1b4770013bcb718

Dr Shilpitha Singh

hi moms nd Dr.  aaj mene 20 th weeks Ka target scan kraya ....nd I have twins so bda baby thik h but chota wala weak btaya h .....nd chota wala bde wale s 1:30 month chota h .....y Kese possible h .....plsss suggest. ...I m so worried