Abdomen measuring small

Jenna • We had our first baby girl September 21 2015 👑 she's our world! Now we're expecting baby number 2 in Sep!!
Went for on ultrasound today they said everything looked good but her abdomen is measuring small so they want us to come back in 4 weeks. 
Anyone have a baby were they were told something was smaller then average? Just curious, I'm not to worried my husband and I are both small.