It depends on the person. I advise my clients who just had a baby to consult with their doctor before beginning any fitness activity. Usually it takes 6-8 weeks after they had their baby, sometimes longer. They have to keep in mind that working out could result into lactic acid accumulation in their blood stream and it can affect their breastfeeding.
Congratulations; remember, the body just went through a huge, miraculous transformation. The doctor clearance is needed first, since the doctor knows all the history. That said, it is usually by the time of the six week mother and baby checkup.
Not to mention, all mothers know that taking care of the newborn is exercise.
Take care and enjoy the babies.
It depends on what you mean by exercise. If all you intend to do during the first six weeks is reactivate muscle memory using gentle restorative movement and breath, then you may start as soon as possible after the birth. As a birth/postpartum doula as well as a fitness and Pilates instructor, I have a program that women can start doing hours after the birth.
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Your client should get approval from her doctor first before beginning to work out again. My wifes midwife told us a month after as long as there are no complications. Also do not expect to start doing all the exercises and duration where a women left off. Once she gets clearance then he should gradually build up exercise by 10 percent a week or less depending on the person. The personal trainer can coordinate with her doctor to get the best and safest workout. Safety is important so there is no rush to start working out until the body returns to normal which is different from case to case.