Expectant mothers will be paid child benefit for their unborn baby from 2009.
Payments will begin in the 29th week of pregnancy, helping parents to cope with bigger bills before and immediately after birth.
On today's child benefit rates, mothers would be more than £200 better off when a first child is born and some £140 better off for subsequent children.
He told MPs he had received "powerful representations" that good nutrition was most important during the final stages of pregnancy.
Mr Brown added: "Help should be available to all mothers expecting a child.
"So child benefit will be paid on that basis for every mother - additional child benefit that recognises the important role at this critical moment that child benefit can play."
The Chancellor announced that benefits paid to the poorest parents will rise to £64 a week from April 2007, compared with £28 10 years ago. And the child element of Child Tax Credit will increase by £80 to £1,845 a year.
Mr Brown said: "Since 1997 two million children have been taken out of absolute poverty and almost one million out of relative poverty."
Nancy Platts, campaigns manager at the National Childbirth Trust, said: "Many of the expenses related to having children start before the baby is born, so this change will provide valuable financial support for parents when it is needed.
"This will particularly benefit first-time parents who may be struggling to meet the cost of a new baby."
However, Mr Brown was warned that more radical action was required if the Government was to achieve its ambition of halving child poverty by 2010 and abolishing it by 2020.
Colette Marshall, UK director of Save the Children, said: "The time for rhetoric is over - we need to start hearing plans on how the Government is going to target the poorest families."
She called for child benefit to be made equal for all children; for low-income households to be given help with fuel bills; and for poor families to be paid seasonal grants at the most difficult times of year.
Mr Brown confirmed that the basic state pension will rise by 3.6 per cent from next April, increasing from £84.25 to £87.30 for single people, from £134.75 to £139.60 for married couples. The pension credit guarantee will go up by £5 a week for a single person and £7.65 a week for a couple.
Mervyn Kohler, head of public affairs at Help the Aged, said: "While the Chancellor painted a picture of a prosperous UK economy, the reality for pensioners is that they are facing the harshest winter for years.
"One in five older people lives below the poverty line - in a country as rich as ours, this is a failure of epic proportions."
Joe Harris, general secretary of the National Pensioners Convention, said: "There was no mention of additional help with the council tax or fuel bills for the majority of older people, and no commitment to tackle pensioner poverty by raising substantially the basic state pension."