- Biparietal diameter of foetus: 21 to 29 mm
- Weight of foetus: approx. 30 g
The placenta now fulfils its function completely: making sure that oxygen and nutrients are transported to the child. Waste products are emitted via the umbilical cord. The placenta also acts as an inner barrier against harmful substances – but unfortunately not against all. For this reason you should be careful of what you eat, drink and breathe in.
In the 13th week the foetus is completely formed. Hair grows in the area of the upper lip and as eyelashes. Even the tiny face muscles function: your baby can yawn, frown, suck its thumb and move its lips. It makes extensive use of its arm and leg muscles too, even if you can’t feel it yet.
The first bones are developed from cartilage tissue; ribs, and leg and pelvic bones are recognizable in an ultrasound image as light ultrasound-dense structures.
Length measurement is no longer reliable, as the child will be increasingly bent forward until the end of the pregnancy. From this point on, its development is judged by head size: the "biparietal diameter" (BPD), the distance between the two parietal bones of the skull, is measured.