Most women don’t even know they are pregnant at 4 weeks. But if you found out this early, you may be wondering “what size is my baby at 4 weeks pregnant?”. The baby size at 4 weeks pregnant is basically microscopic. It is roughly the size of a poppy seed, or about 1 mm in length. You may be surprised to know that your baby’s neural tube, brain, and backbone are already formed! The placenta is beginning to form into protective cushioning for your baby, and many more changes to your body are getting ready to take place.