14weeks is too early for a complete anomaly scan. Even if there are certain negative findings at this stage, you need to follow it up by a repeat ultrasound
which should be done around 18weeks.
Enlarged placenta occurs in a congenital infection called syphilis
and also in diabetes mellitus
in mother. So you can ask your doctor to get additional tests for ruling out these.
Child growth would be affected only in these conditions.
Repeat ultrasound is a must to confirm these findings.