Couples are struggling to have their second or third babies as they are waiting too long to start a family, a fertility expert says.
Travel, careers, and not meeting 'Mr Right' were behind women delaying motherhood, Fertility Associates Wellington medical director Andrew Murray said.
The need for two incomes and the "so-called man drought" had also contributed to the trend of couples having children later in life, after their biological prime.
In the past five years there had been a noticeable increase in couples seeking fertility treatment to expand their brood, Dr Murray said.
Typically, they did not have trouble conceiving the first time even though they were 35 or older, but when it came to trying for more offspring the odds were against them.
"Half of the consultations we're now having are with couples in that position."
One in five couples now faced fertility problems at a time when more and more middle-aged woman were having babies.
A 55-year-old Auckland woman was the oldest to give birth in the past year and the number of women who gave birth aged over 44 was double what it was a decade ago, Statistics NZ figures show.
If a couple had failed to get pregnant within 12 months of trying, they were classed as being infertile. IVF treatment funding is possible for couples if they have only one child aged 12 or under living at home.
Any more children in that age bracket and they will not receive financial assistance for IVF, which can cost up to $13,500 for a single round.
As a woman aged, her egg quality and quantity reduced dramatically, Dr Murray said.
All women under the age of 34 had a 26% chance of becoming pregnant each month. That dropped to 17% for 38 and 39 year-olds, and 8% for women over 42.
Women tended to ovulate their best eggs when they were younger, which was why older women had a greater risk of a genetic problem in their eggs, and why miscarriages and infertility become more common with age.
While the age of the mother was more important than the age of the father, men also had a biological clock, as the number of sperm made each day and their quality declined, Dr Murray said.
"It's really important that it's not just seen as a female problem," he said.
Obesity was also becoming an increasing issue - heavier women did not ovulate as much, and larger men had problems with sperm quality, Otago University senior lecturer in women's and children's health Helen Paterson said.
It also affected their ability to receive government-funded treatment, as women who had a BMI greater than 32 did not meet Health Ministry criteria.