Nigeria's ruling party presidential contender has been flown to Germany for a medical check-up.
Mr Yar'Adua has sought to squash rumours about his health
Umaru Yar'Adua, 55, is the front runner to win April's poll and has previously suffered from a kidney illness.
His People's Democratic Party says doctors had advised Mr Yar'Adua to go for further tests overseas, but insists he will return to the campaign early.
A BBC correspondent says concerns about Mr Yar'Adua's health is giving way to speculation that he may be replaced.
He became the PDP presidential candidate after being backed by President Olusegun Obasanjo - who steps down in April after eight years in power.
A spokesman for the PDP said Mr Yar'Adua's trip abroad was part of a routine medical check-up.
Gen Babangida is being touted as an alternative PDP candidate
The party said he would take time off from a gruelling campaign and would rejoin the campaign trail early.
The BBC's Alex Last in Lagos says the PDP's assurances will do little to calm widespread concerns about Mr Yar'Adua's state of health.
It will increase speculation that he could be replaced as the party's candidate.
Not least, our reporter says, as being president of a country like Nigeria is such a gruelling task and the presidential elections are only a month away.
In January, Mr Yar'Adua sought to squash rumours about his health inviting his critics to a game of squash if they could play 12 straight squash sets.
A tall, thin, quietly-spoken man, Mr Yar'Adua was a relatively unknown northern Muslim governor before he won the governing party primaries with the backing of President Obasanjo.
As the governing party's candidate, with access to its money and power, he is widely considered to be the favourite to become Nigeria's next president in elections in April.
The Katsina governor has a reputation as having run one of the country's most honest state administrations.
Political gossips have suggested a number of alternatives should Mr Yar'Adua be replaced as the PDP's candidate, our correspondent says.
Among them is the immensely wealthy, former military ruler, Gen Ibrahim Babangida.
There are two other strong contenders in the dramatic race for the presidency.
Nigeria's Vice-President Atiku Abubakar is standing for the opposition Action Congress.
But he is currently involved in a bitter feud with the president and is battling on several fronts in the courts to clear corruption allegations which could bar him from running.
The other main opposition leader is former military leader Gen Muhammadu Buhari, who became famous for his war on corruption in the 1980s.
Despite being quite popular, critics have questioned whether he has the money and political machinery needed to win a Nigerian election.
The polls should become the first transfer of power in Africa's most populous country from one elected leader to another since independence in 1960.