Conservative leader David Cameron has said the NHS will be his "number one priority", as the main parties step up their pre-election campaigning.
Unveiling the first part of a draft manifesto, he pledged maternity reforms in England to "meet mothers' needs".
Labour, meanwhile, released a document which they said showed a £34bn gap in Tory spending plans - a claim which Mr Cameron later described as "junk".
Lib Dem leader Nick Clegg said Labour and Tories were "increasingly alike".
The general election must be held by early June but is expected on 6 May, with all the main parties seeking to set the agenda after the Christmas break.
BBC political editor Nick Robinson said a "long, long" campaign had begun, with some "important choices" over government spending priorities "struggling to get through" the rhetoric.
Speaking at the launch in London of the first chapter of the Tories' draft election manifesto, Mr Cameron said childbirth was one of life's "most daunting experiences" and that it was best to conduct this in a "non-emergency setting" where possible.
Before you sigh with weary cynicism, remember that underneath all this, some important choices are struggling to get through
Labour's policies had "given us bigger and bigger baby factories", he added.
Mr Cameron also promised to introduce a "health premium", targeting spending at more deprived areas.
"Health inequalities in 21st century Britain are as wide as they were in Victorian times... We must target resources at the worst-off areas," he said.
Mr Cameron said he would improve the NHS, while tackling the government's budget deficit.
In a subsequent interview with the BBC, the Tory leader said he would not make any "false promises" on spending or tax cuts.
On tax breaks for married couples, considered a policy priority by many in the party, he said he "wanted" to recognise marriage in the tax system and "definitely hoped" that this might be possible during the course of the next Parliament.
But he stressed he could not make a commitment to do this over that period because of the "vast" size of the deficit.
"Today I am not able to make that promise," he said.
He also defended the decision to feature himself in the Tories' new advertising campaign, saying people wanted to know what he would do if he got into power.
"I want this to be a positive campaign about the change we would bring," he said. "We are putting across a positive message."
In contrast, Labour's "negative approach" was focused on attacking the opposition, he claimed.
Chancellor Alistair Darling, hosting what also looked much like the type of media conference seen during official election campaigns, launched an attack on the Conservatives' spending plans.
'Nod and a wink'
He said: "The Tories have made over £45bn of promises, but can barely explain how they can pay for a quarter of this. This leaves them with a credibility gap of £34bn.
"These are not long forgotten promises from another time. All have been confirmed in the last two years. Most have been repeated in the last few months.
"You can't fight an election on a nod and a wink; sometimes claiming you are committed to these promises, and when challenged claiming you are not."
And Health Secretary Andy Burnham said the opposition could not be "trusted" on the NHS as it planned to scrap targets for waiting times for cancer diagnosis and treatment.
Liberal Democrat leader Nick Clegg said: "As far as Labour and the Conservatives are concerned, they will constantly tell people that there are great differences.
"But it seems to me that they are increasingly alike. You have the chancellor, Alistair Darling, who delivered a pre-Budget report where the sums didn't add up and now accusing the Conservatives, because their sums don't add up."
Earlier, Schools Secretary Ed Balls told the BBC he did not know the date of the election, adding that he had "no inside knowledge" of Prime Minister Gordon Brown's thinking about its timing.
But, with some predicting an early March poll, he said he thought it would "probably" come later.
He said Labour would increase the amount of one-to-one tuition for failing primary school pupils in England and that spending on schools would rise "every year, year on year, this year, next year and the year after".
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