Twins baby b measuring 1 week 6 days behind

Jessica

I went for my first ultrasound today and they have seen 2 babies and both have strong heartbeats at around 175 bpm BUT baby B is measuring almost 2 weeks behind.

Has anyone had this happen and the other baby survive? They say to be a tad optimistic as the baby does have a strong heartbeat but that is a huge size difference in my opinion.