16 weeks pregnant

How is your baby growing at 16 weeks?

At about 12cm from head to bottom (crown to rump) and 100g, your baby may have grown by approximately a third since the previous week. Now having additional bone structure in the back and stronger muscles, the body is straighter and the head can be supported in an erect position. The size of the body in proportion to the head is catching up rapidly. The eyes should begin to face forwards, and while your baby still can't control facial expressions, his or her muscles will be getting lots of practice in making them.

The heart is already pumping at a rate of about 50 pints of blood daily. The baby's respiratory and digestive system will continue to develop as the baby practises breathing, swallowing and sucking. Meconium – you should see it as the baby's first bowel movement – may be starting to develop in the intestines.

The baby's nervous system is now developed enough for your baby to be able to clench a fist and hold hands if they happen to touch when reaching towards each other. Your baby will still be moving about to improve muscle development and coordination – and while there's still plenty of room for the baby to move, if you think you felt a slight flutter inside you, that could be your baby moving. This is the first week that some mums-to-be – especially those who have been pregnant before and know what to expect – might first feel their baby's movement. However, it may not be until weeks 18–20 that you notice your baby moving because the early sensations of movement are so gentle.

How are you changing in week 16?

Your womb – and waistline – should now be expanding rapidly. Your womb has grown to half the distance between your pubic bone and belly button, and weight gain of 2–4.5kg (4.5–10lb) is normal for 16 weeks. The placenta should now reach its full thickness of 2.5cm, but it will continue to grow in diameter to accommodate the baby and will weigh about 500g at delivery.

Hormones that relax your muscles and ligaments will make your joints more mobile and increase the possibility of injuring yourself. You may feel a spasm or twinge on your side or a slight pain if you make a sudden movement. This is because the ligaments that are supporting the womb are stretching as your baby grows. As the baby gets larger, the womb has a tendency to sit more on your right side, so you are more likely to experience pain or a cramp on your right side as your pregnancy progresses. It is sometimes referred to as round ligament pain (the ligaments involved are known as round ligaments). Although it is normal to have twinges and some pain at times, if you have pain that continues for more than a few days or that gets worse, talk to your midwife or GP to rule out other possibilities.