DD was almost 3yo. We took her with us to the 12-13 week ultrasound (thinking that she wouldn't really know what was going on anyway) and when the sonographer said everything was looking good then we told DD that we were all looking at her new baby brother or sister. She understood straight away and could actually pick the baby's face from the ultrasound photos! We let her tell our families which was a bit special and made her feel special :-)