Fetal development in pregnancy week 5:embryo in second month The changes to your growing embryo are not quite as drastic this week as they've been in the last few weeks. Growth is now largely focused on their little head, which is starting to develop much more rapidly than the rest of their tiny body. This is because their amazing brain is undergoing some very crucial and rapid development in order to effectively regulate their heart rate, blood circulation, and other vital functions. As for the rest of their miniature body, what were simple limb buds last week are limb flippers this week and the tail is more expressed. Amazingly, within a mere five weeks your little miracle is already developing the rudimentary forms of their liver, pancreas, lungs, stomach and nasal pits while their little heart is already increasing its circulation. Your baby is now a whopping 4-6mm in length. This means our baby is about this big I I (the distance between the two I's is 4 mm)