During pregnancy and finally child birth your body not only undergoes tremendous changes but also is put under a lot of strain and stress in many ways.
The abdominal muscles and the pelvic floor have been stretched enormeously which causes a large amount of women to suffer from rectus diastasis (the seperating of the abdominal muscles in the middle). Start doing your pelvic floor exercises therefore as soon as possible after giving birth.
Whilst it is important to give your body a minimum of 6 weeks resting time to heal and for the abdominal muscle to grow back together, it is equally important to slowly implement a suitable strength exercise routine soon after. This will not only support the core muscles to regain their strength but it also prevents back pain & incontinence. Be careful with overstretching your muscles, due to hormonal influences (especially when still breastfeeding) your ligaments are softened and more flexible than usual.
The best way to implement a suitable exercise routine is to attend a postnatal exercise class under the guidance of a qualified and experienced Personal Trainer.