Week 21 of pregnancy
By 21 weeks, your unborn baby weighs around 350g. They will now weigh more than the placenta. The placenta will keep growing throughout pregnancy, but not as fast as your unborn baby.
They are now covered with very fine, soft hair. The hair is called lanugo. The purpose of this hair is not known, but it's thought it may keep the unborn baby at the right temperature. The lanugo usually disappears before birth.
Week 22 to 23 of pregnancy
Your unborn baby is beginning to get into a pattern of sleeping and waking. This may not be the same pattern as yours. When you're in bed at night, feeling relaxed and trying to sleep, they may be wide awake and moving about.
The lungs are not yet working properly. Despite this, your unborn baby is practicing breathing movements. They are preparing for life outside the uterus. They get all its oxygen from you via the placenta, and will do so until it is born.
Week 24 of pregnancy
Your unborn baby has a chance of survival if he or she is born. Most babies born before this time cannot live. This is because their lungs and other vital organs are not developed enough.
The care now available in baby (neonatal) units means more and more babies born early do survive. There are greater risks of disability for babies born around this time. But there are treatments available. These may help reduce the chance of problems for your baby.