It began as a feel-good story designed to lift America’s spirits during another week of grim economic news.

But last night increasingly difficult questions were being asked about the birth on Monday of healthy octuplets – only the second known case in US medical history – after it emerged that the mother may have undergone fertility treatment, even though she already had six children.

In vitro fertilisation (IVF) treatment guidelines produced by the American Society for Reproductive Medicine say that a woman under 35 should have no more than two embryos implanted at a time. The organisation said last night that it had started an investigation into the case.

Patients must also typically undergo a thorough evaluation before qualifying for IVF treatment.

It was reported yesterday that the family of the octuplets filed for bankruptcy less than two years ago and subsequently abandoned their home. It is thought that they are now receiving government welfare.

Although it remains unclear where, or how, the mother received fertility treatment – she contacted her local hospital when she was already three months pregnant – at least one medical expert yesterday said that if a clinic had been involved it was “extremely irresponsible”. While the identity of the mother has not been revealed, at her own request, it is believed that she is 33 years old and lives with her parents in Whittier, in southeast Los Angeles County.

The marital status of the mother is unknown. Neither is anything known about the octuplets’ father. Another mystery is how the the estimated $100,000 (£70,000) cost of an IVF birth might have been met.

So far, the mother has said nothing apart from releasing a brief statement. It said: “We understand that you are all curious about the arrival of the octuplets and we appreciate your respect for our family’s privacy.

“Please know that in our own time, we will share additional details about this miraculous experience. The babies continue to grow strong every day and make good progress. My family and I are ecstatic about their arrival. Needless to say, the eight was a surprise to us all, but a blessing as well.

“We thank all of you for the positive thoughts, prayers and generosity.”

When contacted at her home by the Los Angeles Times, the octuplets’ grandmother, Angela Suleman, said that her daughter never expected the birth of eight babies.

“They all happened to take,” she said of the embryos, without going into further detail about the procedure. “I looked at those babies. They are so tiny and so beautiful.”

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