Nobody raises an eyebrow when someone makes plans to get married, or to buy a house or to retire. But mention that you are laying out blueprints for a future family and you can find yourself being considered a bit clinical at best and down right cold-blooded at worst.

Yet many couples who try to juggle careers and family admit that if they could turn back the clock, they would have spent more time trying to work out the potential cost factors involved in starting a family and in finding ways to compensate for the drop in disposable income that would follow.

One of the biggest shocks of having a baby after the sleepless nights and huge sense of responsibility that descends is the impact that this tiny person can have on the family income.

In the space of a few months, even a well-paid, dual-income professional couple can go from having several hundred pounds of spending money each month to being "flat broke and wondering where all our money disappeared", as one couple recounts.

Having a baby isn't necessarily going to drive you into serious debt, but it often means that you can no longer sustain a childless lifestyle, which often includes regular dinners out, two annual holidays and spontaneous wardrobe additions.

The first thing many new parents notice is how the grocery bill rises steadily: between milk formula and brand name nappies there is an automatic £15 extra tagged on per week. Once baby starts eating solid foods, at least another £5 is spent on jars and cereals. That extra £20 a week over a year amounts to £1,040.

Despite the generosity of grandparents and friends, other purchases for the baby, such as clothes, furniture, prams and car seats, toys, books etc can easily cost another £500. Add another £100 for extra health insurance, GP bills and medicines.

But most of all, there is the cost of child care.

Let us use the example of a couple aged 31 and 29, earning a substantial total salary of £50,000. They live in a two-bedroom terraced house in the inner city which was purchased four years ago for £70,000 with a mortgage of £60,000.

Monthly repayments, including mortgage protection and building/contents insurance amount to £515. His monthly take-home pay after personal allowances, pension, health insurance and mortgage tax relief allowances and income tax are taken into account, amounts to £1,760. Her after-tax income (based on her £20,000 salary) is proportionately lower at £1,000 a month because he has taken the major allowances, like the mortgage interest and VHI relief.

Out of the £2,760 a month, the couple pays £515 for the mortgage, £500 in car repayments and petrol for their two cars, £200 in AVC pension contributions, £150 for utilities, £500 for groceries and eating out, £200 for entertainment, £50 for life insurance, £160 for motor insurance and tax, £200 savings in a building society and the balance of about £285 (or £3,420 a year) on discretionary items like clothes, furnishings and holidays. Credit card statements are cleared each month and overdraft facilities are seldom used.

The arrival of their baby costs an additional £1,600 at the very least, or an extra £133 a month. But it was the extra £65 a week in creche fees (£280 a month, £3,380 a year) in the first year and the wife's decision in year two to take a career break which pushed their incomes and budget to the limit. There is no ideal way to climb the career ladder with a mortgage and children also hitching a ride, and many women decide to interrupt their career when they have a baby. The year that the woman stopped working, the couple's take-home income fell by nearly £8,000 (when the personal allowances were adjusted) and while the child care bill was no longer a problem, they were effectively now living on £1,950 a month, but with outgoings of about £2,500 which did not include discretionary spending on clothes, furnishings and holidays.

Selling the second car would save this couple £150 a month in petrol and insurance, but only if the husband walked or cycled to work. Cutting the building society savings back to £50 from £200, reducing his AVC outgoings by a third and cutting out their extra entertainment allowance would save about £700 a month, enough to meet their outgoings and have a couple of hundred pounds extra for their discretionary spending on the baby, their house and a family holiday.

Few people in this sort of financial situation would regret having a baby, especially since they do not have any serious debt to contend with and their financial position would automatically improve if the wife returned to work in the future.

As any couple with small children can attest, the best laid plans can go awry. The woman may have every intention of returning to work full-time but child care problems, health and stress issues can persuade the most fervent careerist to job share or give up work altogether for a few years.

Ideally, the tax and income repercussions need to be considered before such a change.

Spending habits also need to be seriously examined: a baby will play havoc with a couple's social life and will eliminate a great deal of discretionary spending that will be replaced by essentials like nappies, formula and gripewater. Money that was once saved is now going to be needed to meet the extra costs, especially of child care or as an income supplement if a spouse stops working.

Another possibility that needs to be considered early on is the size of a mortgage. Taking on a huge home loan when there are two incomes coming in may seem realistic, but many couples finally get married in order to have children and it is not unusual for one partner to move out of full-time work at some stage.

Before buying your mortgage you should discuss this possibility with the lender and find out if it is flexible enough to accommodate payment breaks and reductions.

You need to seriously look at the size of your property. A bijou cottage in the Liberties may be perfect for just the two of you, but will soon fill up with baby stuff. Can you afford to move and have a baby as well?

Finally, don't scrimp on life insurance it is cheapest when you are young and is a priority if you have a young family. Try to keep putting a small amount into a savings fund: school/college fees are just around the corner.