The case of Keli and Tegan Lane is the most baffling I've ever come across.

I can't say Keli Lane didn't kill her child — but I can say she doesn't fit easily into the categories of those who do.

Over a 30-year career as a perinatal psychiatrist, I have been asked to assess whether mental illness was a contributing factor to infanticide — the killing of a child under one year by their mother, or neonaticide if under 24 hours — in maybe a dozen cases and read research of maybe a hundred or more others. Keli doesn't fit easily in this company.

I saw Keli Lane twice at Silverwater Correctional Complex in NSW. I went to see her in order to help make sense of why and how — even if she is innocent, as she claims, of Tegan's murder, there were still five hidden pregnancies before keeping the sixth child — and why Tegan was not adopted like two others.

Keli was never my patient and she confirmed that she knew all that she said to me could be made public. There was no mental health defence at her trial, and she is competent psychiatrically and legally.

Professor Anne Buist visited Keli Lane at Silverwater Women's Correctional Centre on two occasions. ( ABC )

Visiting Keli in prison

Like all prisons, Silverwater is an imposing structure — huge, covering a vast area of land. Keli isn't a risk to anyone, so relative privacy was granted. I've taken a pen and paper, that's all I'm allowed to take in.

Keli is a slim, attractive woman in her middle years, dressed prison-plainly, who gave a cheerful greeting to the guards. She was wary at first, but warmed up and was both engaged and engaging. She had the demeanour of a younger sister's girlfriend who you vaguely knew, chatting in the backroom at the local RSL club.

I had read the documentation of Keli's case and viewed the video of the police interviews. Tegan (the fourth pregnancy) went missing within two days of her birth, after leaving with Keli from the obstetric hospital.

The timeframe is technically outside the definition of neonaticide, but like the other neonaticide cases in reviews and that I have seen (or attempted neonaticide — in several the babies survived), Keli's babies were unplanned, unwanted and unacceptable.

Also in keeping with this subgroup of infanticide, she was young, with a pre-teen level of maturity and a family that didn't talk about emotions. She didn't realise she was pregnant until late; for these girls, this denial can be at a deep psychological level or actively pushing it out of their mind when it surfaces. It's difficult to imagine the shock of childbirth under such circumstances. They panic; hit out in fear, run, and may have no recall. These are familiar reactions to severe stress and trauma, and some courts will accept them as constituting a mental illness (acute stress disorder).

But in Keli's case, this is where the story differs. There was no evidence of the "panic" at delivery, typical of neonaticides. She knew she was having a baby by the third trimester, and delivered in a hospital. She planned what to do in all her pregnancies — two terminations; two adoptions; and in Tegan's case she claims she gave to the father of the baby and his girlfriend. She planned around her commitments on all occasions, making up stories if she needed to hide where she had been.

Keli Lane gave birth three times in secret, without any support from friends or family. ( ABC: Supplied )

There was no compelling reason she had to be at the wedding she turned up at after Tegan's birth — she could have made up any number of stories as she had in the past. That she did turn up was in keeping with her character; you pulled yourself up and got on with things.

Keli did not have psychosis: psychotic and severely depressed mothers whose judgment is significantly impaired may kill their children for altruistic reasons. They often commit or attempt suicide. Other mothers who commit infanticide have less clear diagnoses (and personality disorders are not considered mental illness for the purpose of defence). They are often isolated, unsupported and desperate. They frequently love their baby but struggle to keep their child in mind as a result of their own childhood trauma, drug use and need for partner's approval. She didn't fit this group either.

Was she a narcissist?

The psychiatrist who reviewed Keli's police videos for the prosecution but didn't interview her, concluded that she had a narcissistic/psychopathic personality disorder.

I don't agree.

Keli has some narcissistic traits, but doesn't tick enough boxes for a diagnosable disorder. Personality disorders are lifelong patterns, whereas her psychopathology was from age 16 to 24. Outside this time, she has no significant history of illegal behaviour or irresponsibility. She has maintained friendships, kept a job where she was liked, and sustained a marriage of several years. She has a current partner and regular contact with her daughter who she gave credible evidence of caring deeply about.

So why these hidden pregnancies, and what was it about her environment that allowed or supported this to happen?

And would this explain why Keli adopted out two children but, according to her conviction, murdered the one in between?

Keli's answers to my questions strongly suggested an avoidant attachment style, which is characterised from infancy by the child recognising that their caregivers are not comfortable sitting with and managing negative emotions. From a very early age, Keli got the message that her parents didn't like dealing with negative emotions. She learnt to suppress her emotions but still craved connection.

Keli Lane's parents, Rob and Sandra. ( ABC: Supplied )

Keli's parents were heavily involved in sport, and she found her connection there. Her father was her role model and she became "one of the boys". She was strongly built, drank heavily, didn't bother with makeup.

There was little space in her teenage life of pre- and after-school training (at times 4.30am to 9:30pm) to reflect on becoming a woman. Her many coaches reinforced what Keli already knew to do — focus on sport. This also reinforced the underlying message — success (at sport) is what is rewarded.

'One of the boys'

There was little time and no encouragement for mother-daughter chats or giggling with girlfriends over boys and makeup and how tedious periods were. At important milestones — menarche, first sexual experience, first pregnancy — Keli felt traumatised, largely because she was unprepared or because of other factors happening at the time for her family and friends, and felt unable to seek support.

Here is the psychological immaturity I had seen in other neonaticides. But there were differences; in particular, why so many pregnancies?

In Keli's teenage years there were parallel processes at work.

File photo of Keli Lane running on the beach in a competition. Keli grew up in a sporting family. ( ABC: Supplied )

The first was a need to be loved and accepted by her family. From her perception, there was no role model or encouragement to be anything other than outstanding at sport. Even when awareness of a pregnancy bubbled to the surface it didn't stay there long: she used the method of dealing with anything that caused anxiety, anger, fear or sadness that had been constantly reinforced since she was a baby — denial and repression.

The second process was her development from being "one of the boys" to a sexually mature woman.

It took six pregnancies for the latter process to win over the former. Keli's psychopathology was centred in that time in her life. Because she was never called out on any of the pregnancies (or at least not in a way she couldn't bluff past), she kept doing what she always did, the only thing she knew how to do with the tricky emotional female stuff — squash emotions down, get on and be successful.

Sorry, this video has expired Caro Meldrum-Hanna talks to Kelli Lane

Murder doesn't fit with Keli's image of herself

Though her planning was neither brilliant nor deeply considered, Keli planned what she was going to do in every pregnancy. She had been given a hard time with the social worker's questions at first adoption; being made to think about the child was harder still. Conceivably she was looking for an easier option with Tegan. In the following pregnancy because there was no-one to give the child to (Keli's story) or because killing Tegan had traumatised her (the prosecution psychiatrist's theory) she returned to the option of adoption.

But would she plan to kill a child?

It doesn't fit with any version of herself she has ever had, and is out of keeping with her other behaviour.

Lying to escape scrutiny is a long way from murder.

It is just as conceivable that Keli left Tegan somewhere after they were discharged from hospital hoping, perhaps unrealistically, she would be found, but I find it far less believable that she bashed, drowned or suffocated the baby.

These are acts of panic, not planning. And not acts of someone who saw herself as doing the right thing under difficult circumstances on each of the other occasions, who was upset by being forced to see the child prior to Tegan, and who continued to finally mature to a stage where she could keep a child, and be a mother.

In the past, even in the years of her problematic behaviour, Keli was concerned to provide the best outcome for the child within narrow childlike limit of thought, by adopting them out or — ostensibly — giving them away. This would include someone (father of the child or otherwise) who said they wanted a child, no paperwork or questions asked.

Though the story as it stands has holes, the concept fits with her psychopathology, which murder does not, however more complex it is than the average case of infanticide; from a psychological perspective, at the level of reasonable doubt.

Anne Buist is Professor of Women's Mental Health at the University of Melbourne and Austin Health. Follow the investigation in the Exposed ABC Facebook group.