IVF Babies

Just wondering, does anyone know if babies conceived through in-vitro fertilization develop in the womb slower than babies conceived the natural way?  Had an ultra sound the other day and was told my baby looked small for being an IVF baby.  Told not to worry because the heart beat was strong, and I am only around 12 weeks along. They also mentioned at the clinic I went to that IVF babies usually have a lower birth weight, but everything up until now has been good.  My last ultra sound was right on track for where I was supposed to be.