40 weeks pregnant, or 38 weeks after conception: difficult to estimate the weight

Your baby could weigh 3.5 kg and measure about 50 cm. However, this is only an average. Smaller babies can weigh 2.5 kg while larger babies could be over 4 kg. The estimate of fetal weight at this term is not perfectly accurate and may vary by more or less 10%. 

Your baby's head circumference, averaging 9.5 cm, is the largest part of their body. The fontanelle, soft membranous gaps between the cranial bones of a newborn's skull, allows flexibility as your baby passes through the pelvis and gradually ossify as they grow.

The brain weighs between 300 and 350 grams and continues to mature for many years after birth.