Pregnant women will get about £200 paid into their bank account to spend on healthy food under a government initiative, the BBC has learned.
Nutrition in pregnancy is very important
From April 2009, expectant mothers in England would receive a one-off payment from their 29th week of pregnancy to encourage them to eat well.
Health secretary Alan Johnson will announce the "Health in Pregnancy Grant" on Wednesday.
Charities welcomed the idea but warned it may not be effective.
For example, there is no measure to ensure the cash is spent on healthy food and babies also need good nutrition before 29 weeks.
The grant's aim is to cut infant mortality and close the health inequality gap.
Infant mortality rates have done down, but more so among affluent sectors of society than poorer socioeconomic groups, which is widening the health gap.
By 2010, the government wants to see a 10% reduction in this gap from the baseline year of 1998.
Experts know that poverty and nutrition play a role in pregnancy health, and it can be difficult to eat healthily when on the breadline.
Women on a poor diet during pregnancy are more likely to have low birthweight babies.
That increases their baby's risk of poor health during childhood and during their adult life.
A government spokeswoman said cash was a "key determinant" when it comes to good health and healthy eating in expectant mothers.
Mothers-to-be on low incomes can already apply for a £500 grant from 29 weeks of pregnancy to help towards the cost of having a new baby.
The Health in Pregnancy Grant would not be means-tested.
To get the payment, however, a woman would have to meet with a health professional for tips on pregnancy health and welfare advice.
A spokeswoman for the National Childbirth Trust said: "This proposal sounds good in principle - nutrition in pregnancy is very important.
"However, the logistics of the scheme need to concentrate on ensuring that the funding directly benefits women and their babies nutritionally."
Amy Edmunds, spokeswoman for the premature baby charity BLISS, said: "Babies most in need of support are those born before 29 weeks' gestation.
"Around 12% of premature deliveries last year happened at 29 weeks or earlier, which suggests that many women could miss out on the new grant.
"Poor diet is only one of many potential factors that contribute to premature birth and low birth weight.
"We recommend that the government addresses the fact that the service that cares for these vulnerable newborns is chronically under-resourced."
A Royal College of Midwives spokesman said: "We are seeing a real and worrying widening of the health inequality gap so any move to improve the health and wellbeing of pregnant women and their unborn child is a positive step.
"However, to really tackle health inequality we need a real, long term and committed focus on public health with a significant increase in resources.
"Improving the health of women and children is the work of decades, and let us hope this is a step towards this."
Shadow Health Secretary Andrew Lansley said: "It is vital to improve public health but it can¿t be done with one-off gimmicks.
"This proposal does not appear to result from evaluation of any pilot scheme.
"For the government now to be talking about enhanced interventions with mothers in deprived areas is deeply hypocritical when the number of midwives hasn¿t kept pace with the increase in births over the last five years."