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Your baby's development
By 21 weeks, your baby weighs around 350g1. 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 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's born.
By the time you're 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 aren't developed enough.
The care that can now be given in baby (neonatal) units means more and more babies born early do survive. But for babies born around this time, there are increased risks of disability.
Your body at 21-24 weeks pregnant
Your womb will begin to get bigger more quickly and you'll 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 22-24 weeks pregnant.
They may appear on your stomach, breasts and thighs. At first they look red, then fade to a silvery grey. Your breasts may start to leak a little pre-milk – this is normal.
1 Contains public sector information licensed under the Open Government Licence v3.0.