By 21 weeks your baby weighs around 350g. From about this stage onwards your baby will weigh more than the placenta (which, until now, was heavier than your baby). The placenta will keep growing throughout pregnancy, but not as fast as your baby.
Around this time, the baby becomes covered in a very fine, soft hair called lanugo. The purpose of this isn't known, but it's thought that it may be to keep the baby at the right temperature. The lanugo usually disappears before birth.
Your baby is beginning to get into a pattern of sleeping and waking, which won't necessarily be the same as yours. When you're in bed at night, feeling relaxed and trying to sleep, your baby may be wide awake and moving about.
The lungs are not yet able to work properly, but your baby is practising breathing movements to prepare for life outside the uterus. Your baby gets all its oxygen from you via the placenta, and will do so until it is born.
By the time you are 24 weeks pregnant, the baby has a chance of survival if he or she is born. Most babies born before this time cannot live because their lungs and other vital organs are not developed enough. The care that can now be given in neonatal (baby) units means that more and more babies born early do survive. But for babies born at around this time, there are increased risks of disability. Find out more about premature labour and birth and special care for babies.
Your womb will begin to get bigger more quickly and you will really begin to look pregnant. You may feel hungrier than before – try to stick to a sensible, balanced diet, and make sure you know what foods to avoid.
Not everybody gets stretch marks, but if you do develop them they will probably start becoming noticeable when you're around 22 to 24 weeks pregnant. They may appear on your stomach, breasts and thighs. At first they look red and then fade to a silvery grey. Your breasts may start to leak a little pre-milk – this is normal.