Is it enough for you if the baby is healthy, or must it be good-looking too?" the woman asked, her tone confident and alarmingly businesslike.

The issue of appearance would reappear in the conversation. Along with weight. Sex.

Price.

For example, the woman told her caller, a man from a northern Tamil Nadu town, that his most expensive option would be a boy with skin like an "Amul baby" -- in other words, a fair complexion.

And although she mostly used simple numbers like "4" or "4.25" to talk about payments, it was clear this was shorthand for larger amounts.

"I've been doing this for 30 years. By God's Grace, I haven't faced any problems." - Woman in audio clip

The audio surfaced on Wednesday night on social media. When the news spread, Tamil Nadu's health secretary asked the collector of Namakkal, the woman's district, to investigate. Police detained her for questioning in Rasipuram, the district headquarters.

Puthiya Thalaimurai, a Tamil channel, said on Thursday evening that she and her husband had been arrested.

The woman is a former government hospital nurse who took voluntary retirement. She admitted to the police that she sold three babies in Rasipuram.

'30 YEARS'

In the audio clip, a retired nurse speaks to a man from Dharmapuri. The man said he and his wife had been childless for seven years. Their conversation (in Tamil) was unnerving not just because it was about the sale of infants, but also because the woman seemed to describe corruption in local government.

While the woman asked her caller to meet her in person and make an advance payment, he was interested in knowing how much he would need to spend. At one point, she appeared to say she would obtain signatures from the baby's parents and then asked the man whether there would be trouble from suspicious neighbours.

"I've been doing this for 30 years," she said. "By God's Grace, I haven't faced any problems."

At the start of the audio clip, the woman appeared to say that a baby the man had previously wished to buy had been sold to someone else.

The local adminstration got a mention when the woman said she could get an "original" certificate -- presumably a forged birth certificate -- from the municipality in a few weeks, for a separate fee of Rs 70,000.

"The baby can even be taken abroad. Original certificate." - Woman in audio clip

"It's all municipality people," she explained when the man softly asked how much the certificate would cost. "They do it by committing fraud. Everything is [on the] Net now, right?"

"The baby can even be taken abroad," she said later.

"Original certificate."

PRICE

When the duo discussed costs, the woman was extremely specific -- she explained how the weight, sex and complexion of a baby could affect the price. Keep in mind that the numbers below surely represent large amounts of money.

"If it's a female baby, then arrange for [an amount] upwards of 2.70," she said. "If she weighs three kilos, they'll say up to 3 rupees."

And if the man wanted a boy?

"If he's black [has a dark complexion], I'll get it done for you for 3.5 to 3.70."

A fair infant with skin like an "Amul baby", she said, would cost the man "4, 4.25".

Investigators are trying to determine how many children were sold and whether there is truth in the woman's claims that she has sold babies for 30 years.

They are also looking into her claim about birth certificates, and are working to find out if the parents of the babies were coerced, or if there was a mutual agreement.