A district judge has urged lawyers to be sensitive about casting allegations of mental illness against a party in their litigation strategy.

The plea came in a rare postscript to a ruling on a custody tug-of-war between a split couple over their toddler son.

District Judge Kathryn Thong said a lawyer should not claim that a spouse has a mental disorder to boost his own client's case, noting that a wife's mental condition in itself does not disqualify her right to care and control of the child.

"Rather, it is the effect of such mental conditions and how it correlates to a child's welfare," said District Judge Thong.

She added in judgment grounds earlier this month that if a party argues that his spouse has a mental disorder or is pre-disposed towards one, then he should take extra care to avoid worsening or triggering the condition.

"Waging litigation upon litigation under the banner of 'best interests of the child' might actually undercut the (father's) case, for how does it advance a child's interests for a parent's mental health to suffer through unending and tedious litigation?" she added.

The case involved a 49-year-old senior executive on a $25,000-a-month salary who had married a now unemployed 36-year-old bankrupt in October 2016 after a whirlwind romance.

Their son was born in March 2017 but trouble brewed early in the marriage and the mother showed unusual behaviour such as cutting up photo albums, smashing crockery and wielding a pair of scissors in front of the father.

It later emerged that the father "had some inkling then that the mother could be extreme in her behaviour and was on medication", the court heard.

He was aware that the woman did not have a stable family background or any social network, but his desire to have a child did not let him act on those warning signs, noted a court-appointed psychiatrist.

Matters escalated and on Jan 11, 2018, the father suggested to the mother that she have a shower and then go for a family walk.

But while she was in the bathroom, the father, the child and the domestic helper left the matrimonial home for good.

That day, the father's then lawyers wrote to the mother stating that in the interim both parties could have joint custody of the son with sole care and control going to the father. She could see the boy only on a supervised basis.

Both parents that same month took out individual applications under the Guardianship of Infants Act in relation to custody, care and control of the now three-year-old boy.

The father, represented by lawyers Foo Soon Yien and Brenda Kong, argued that the mother had a mental condition - borderline personality disorder (BPD) - that needed to be assessed.

After having uncovered her patient history and how many psychiatrists she had been visiting and the medications she obtained, the father demanded that the mother be allowed supervised access only.

He argued that the boy could be in danger as the mother was not only emotionally unstable but the large doses of medications also suggested side effects.

The mother through her lawyers, Mr Imran Khwaja and two others, countered that she merely had BPD traits and it was a leap to suggest that she had BPD itself.

All she had was social anxiety, they added, and not a mental condition that would prevent her from caring for the child. They said her ability to withstand the constant pressure of the legal proceedings was reason enough for the court to find that nothing was impairing her ability to care for the boy.

District Judge Thong ruled that the couple should have joint custody of the child, but care and control should go to the father.

She held that the boy's best interests would be served with the father, who had a domestic helper.

The judge found, among other things, the mother's dependency on medication suggested that she could not manage herself, despite being an "intelligent and resourceful woman". She also noted that the mother did not follow up on the interim court orders granting her supervised access.

District Judge Thong added that without her lawyers, it would have been much harder for the mother to "withstand the pressure of litigation". "As divorce proceedings are afoot and contested, I hope good sense can prevail on both sides and parties can soon move on with their lives, whilst co-parenting the boy."

The father is appealing against joint custody and the mother is cross-appealing against care and control for the man.