You are now 4 weeks pregnant (14 days after conception), which is the beginning of week 5. This is when the woman’s next period would have been due (and is now late!) The blastocyst is now called an ’embryo’, and your baby inside measures about 1.5 mm (or 0.06 inches). On day 18 after conception, your baby’s spinal cord and brain (called the ‘neural tube’) begin to form. The top layer of cells (or the ‘ectoderm’) forms the back of the embryo. It is a flat sheet of cells that folds in the middle and curls around to start forming a tube (which will house your baby’s spinal cord). The top end of the tube balloons out and enlarges to form the beginnings of your baby’s brain (although it is quite primitive at this point). By the end of this week (day 21 after conception), the 2 sides of the ‘sheet’ meet and fuse to complete the neural tube.
At the same time, the endoderm (or bottom layer of cells) balloons out the front of your baby at chest level, to form a yolk sac and a body stalk protrudes from your baby’s waistline. The body stalk is the beginning of your baby’s umbilical cord that will nourish them once the placenta is fully developed. In the meantime, the yolk sac provides some nourishment and is eventually reabsorbed into your baby’s body to form their bowel, liver, lungs and bladder. The yolk sac is also where the baby will start producing their own blood supply.
The earliest time possible to detect a pregnancy with ultrasound images is about 5 ½ weeks. A tiny sac can be seen, but the baby and their heart beat are not able to be detected yet. Sometimes a pregnancy of this stage is missed by an ultrasound, because the sac is so tiny.
