At the Salvation Army's Bethany Centre for young mothers, two girls sit side by side. Their hair is pulled back neatly from their foreheads and their schoolgirl complexions glow with youth. It's difficult to believe the warm, snuffling bundles in their arms belong to them.

Justlin is 16. Crystal - a second-time mum - is 18. Neither of the girls planned to be mothers so young. "I knew about contraception," Crystal says. "We all got taught about that at school. But then your partner doesn't want to use it and you think it won't happen to you."

Justlin never thought much about it. "Before I got pregnant, I was just a typical teenager, out and about, doing drugs ... living at home."

Neither woman has any qualifications. The fathers of their babies are not in a position to support them. Justlin describes her baby's father as a "reforming criminal". Crystal says the father "doesn't believe the baby is his".

Both girls adore their babies, but the road ahead is one of uncertainty. "I think I might like to be a flight attendant," Justlin says. "But, I don't think I will be able to fit that around a baby. It's just a dream."

Across town, at the Mt Eden Plunket rooms, Sandra Law tells a very different story. At 37, she has just given birth to her first child - Cooper, aged three weeks.

Law didn't want to have children in her 20s or early 30s, let alone her teenage years. She was too busy getting an education and travelling the world, working in a variety of occupations.

"I hadn't found the right partner and it's a huge responsibility that I wasn't prepared to enter into without the right person. That was a compromise I wasn't willing to make."

Technically, Law is old enough to be the grandmother of Crystal and Justlin's babies. In medical speak, she is an "elderly primagravida" - a rather alarming term for a woman over 35 giving birth for the first time.

For Law, however, age is simply not an issue. "Physically I felt I could leave it to this late because I have always been quite fit."

She became pregnant immediately after stopping the contraceptive pill. Her baby was born after just four hours of labour, weighing in at a healthy 4kg (9lb).

Law plans to return to work after three or four months. She will share the childcare responsibilities with her partner and her mother.

Law's story could not be more different from that of Crystal and Justlin. Yet, together they represent the statistical blips which make New Zealand mothers stand out on the international scene. On the one hand New Zealand has the third-highest adolescent birth rate in the industrialised world (7 per cent of all live births last year were to women under 20). Only the United States and the Russian Federation have higher rates.

On the other hand, the average age of first-time motherhood in New Zealand has consistently been among the oldest in the industrialised world for at least a decade. In other words, if we don't have our babies very young, we have them when we are - comparatively at least - very old. Last year, the 30-34 year age group was the most common age for childbearing. Just over half of all newborn babies had a mother over 30.

Plunket's general manager of clinical services, Angela Baldwin, is uncertain why New Zealand has two such disparate trends of motherhood. But, in part, she believes it is because of differences in access to education.

"I guess it is [a sign of] the growing disparity between groups within New Zealand. But we obviously have an acceptance [of teen pregnancy] within the community."

If you ask Crystal and Justlin why they got pregnant, it is difficult to get a simple explanation. Both say they didn't want to get pregnant so young but neither believes anyone could have prevented it happening.

Both dropped out of school early and started mixing with a "bad crowd", drinking and doing drugs. As Justlin explains it: "I had an attitude problem. My family would sit me down and talk to me about it [contraception]. But I was like, 'Nah, it gets in the way'."

Older mums, on the other hand, can usually give a comprehensive explanation why they got pregnant. Often they are well-educated and have delayed having children while they establish their careers, pay off their student loans and find the right partner.

Says Baldwin: "Whereas, 40 years ago, marriage might have been choice number one, today's women might think, 'Yeah that's something I may do sometime'. Women have different ambitions and of course have more choices."

Law, for example, delayed while finding the right man. "It comes back to that ultimate challenge of the person you want to breed with."

Auckland senior lecturer Debbie Payne, an older mother who has researched the subject, believes this is common.

Like Law, she had her first baby at 37. She was in a stable relationship and had a Master's degree (she has since gained a doctorate) and many years of work experience and saving behind her.

"The reason why I delayed it, I guess, was finding the right partner and circumstances. It's not just getting yourself economically settled or advanced in your career."

Giving birth at either end of the age spectrum has its pros and cons. Young mothers may have more energy but are usually less financially and emotionally secure. Older women, on the other hand, face stereotypes and even prejudice about the risk of older motherhood.

Medical literature argues first-time mothers over 35 are more at risk of complications during pregnancy, labour and after the birth. The incidence of conditions such as Down's Syndrome also increases as women age.

Dr Payne believes the risks are sometimes unnecessarily overstated. "There is another way of thinking about maternal age, where it doesn't necessarily identify maternal age as a health issue. I found there were practitioners and women who felt that because these [older] women were fit and healthy, had no underlying diseases and had good nutrition and a good environment, that they had the potential to birth quite normally."

As far as Plunket is concerned, there is no ideal age of first motherhood. Baldwin says many older mothers - particularly those in their 40s - are very conscious of their age and consequently take greater care of themselves.

"I guess it [the ideal age] is when the woman is in a supportive relationship - whatever that is - and is as fit and able as she can be, and she wants to have this child."

Adjusting to parenthood can be difficult at any age, Baldwin says. Some older women who have had control of their lives for a long time may find it difficult. But then, they often have more resources and know how to get support.

Young mothers, particularly those with transient and unsettled lifestyles, are at greater risk of alienation from support networks. Though New Zealand has many good initiatives to help teen mums - including places like the Bethany Centre and units attached to schools - it can be difficult reaching the most isolated, at-risk teenagers.

When Crystal had her first baby, she and her boyfriend initially tried to cope on their own. "The hardest thing was getting up all hours of the night and getting tired because I didn't get much sleep, and keeping the housework under control and keeping my man happy as well as my baby."

Crystal developed post-natal depression and her relationship ended a few months after the birth. Her mother agreed to look after the baby on the condition Crystal went out and got an education.

The plan backfired: "I didn't feel I had had my teenage years of fun," Crystal says. "And when I went out to have fun, I got pregnant again."

She considered adoption, but as soon as she saw her new baby's face, she decided against it. "I thought, I could never give my guy away. I thought, I can do this."

This time, she plans to move into a small flat by herself. Her first priority, she says, is her baby. The second, to get an education.

Justlin thinks she will probably go to her grandmother's house when she leaves the Bethany Centre. Her boyfriend, at present on probation, is hoping to help support her once he sorts out his own life.

Of all the differences between the older mothers and their teenage counterparts, perhaps the greatest is that the older mothers believe they have had their babies at the right time. The teenagers, however, wish they had waited.

"I want to tell other teenagers not to do it," Crystal says. "Don't be like us. You won't keep your boyfriend. They [teenage girls] think they are pregnant and they will keep their guy doing that, but it's not true. Guys don't want the responsibility sometimes. They run off. It's not fair on the child."

Justlin chimes in, sounding more like a guidance counsellor than a 16-year-old. "It's no picnic in the park. You never sleep, you forget to eat. Get a stable relationship and stay in school."

Crystal can't imagine having babies in her late-30s, though. "I would rather now. Thirty is quite old."

Justlin is more circumspect: "They are old, but they are more secure. In your 20s is ideal."

Crystal thinks for a moment. "I wouldn't mind if I was twentysomething, married and had a good job," she says. "Not too old though, I reckon."

* The teenagers' names have been changed for family reasons.