Being overweight or underweight to a certain extreme will reduce the ability to get pregnant. Same goes for men's sperm count. Also, the hormone fluctuations during and after pregnancy are NOTORIOUS for causing weight gain, so women have to be extra-diligent during these times to stay a healthy weight.
If she doesnt watch it now after child birth it will be really hard to lose it all. Having children puts a toll on a women's body. Keep fit and it will be easier to get back into shape after a baby. Not everyone is blessed to look like a supermodel after children.