Other things to consider are the type of maternity package offered by your employers - usually 90% for six weeks and then SMP for nine months, together with the benefits you will receive once the baby is born - child benefit, working families tax credit, child tax credit, and additional costs of childcare.
Your IP will expect you to have checked this out firstly, so that he/she can consider what impact any income/expenditure fluctuations will have on your ability to make your payments. Hopefully a variation may not be necessary, with careful planning and budgeting, but it does really then depend upon whether you intend to return to work or not.
Children bring different priorities to our lives, and it may be that having a family becomes far more of a priority for you than paying off your creditors. So if money is going to be very tight, also consider bankruptcy proceedings as Ian has already advised.
Regards, Melanie Giles, Insolvency Practitioner