Demographers tend to be control freaks who get nervous if the population rises above or falls below some ideal benchmark -- zero growth, for example. But people tend to procreate -- or not -- with reckless disregard for demography. In Britain the average birth rate fell to 1.63 in 2001, but since then it has leapt to 1.96 (2008) -- nearly back to “replacement” level.

For the first time in a decade, the birth rate last year played a bigger part than net migration in overall population growth in the UK, The Guardian reports.

Undoubtedly the biggest factor in the present boom is immigration. There are more women of childbearing age in the UK and they have come from the Indian sub-continent, from Africa and from eastern Europe to work here and make homes here. The largest number of non-UK born mothers is from Pakistan, but strongly challenged now by Poland. In 2005, there were 3,403 births among Polish women in the UK. Last year there were 16,101.

In total, just over 24% of births were to women born outside the UK, making them part of the baby boom but not by any means the whole story. The number of births in England and Wales increased by 6.5% to non-UK born women, but also by 1.5% among those born here.

To get a handle on the current baby boom The Guardian visited the small town of Boston, located in a vegetable and fruit growing area in Lincolnshire. Here the total fertility rate (TFR) is 2.8 children per women -- twice the rate of some areas. Among the factors encouraging births are lots of immigrants, young couples, affordable housing and new transport links, and the background farming culture. Maybe there is something catching about having children when lots of other people are doing it.

Elsewhere career women who have been suppressing their fertility are now having babies -- over the past five years the highest fertility rates have been among women aged 30 to 34. In vitro fertilisation has played a minor part in the overall scheme of things.

David Coleman, a professor of demography at Oxford University, says delaying childbirth has a long history in the UK. For centuries this was done by delaying marriage or not marrying; today it is achieved largely by the use of contraception (although this is accompanied by a trend towards later marriage as well). The big baby boom of last century was driven by early marriage, Coleman suggests, since, even with “comprehensive contraception” available, early marriage tends to produce more children. But women’s liberation (into the workforce) put paid to that and from the 1970s they began to put off having babies.

The new trend has been encouraged by the UK government with working families tax credits, a child tax credit and an increased child benefit. Paid maternity leave went up from 18 weeks to 26 and then 39 weeks and is set to rise to a year. Child care is also subsidised.

So, where to from here?

Coleman says he is one of the rare demographers who believes we may yet get back to a replacement fertility rate. It is countries like Italy that have a bigger problem. Where the ties of kinship are very strong, unlike in the UK where people know few relatives and are unlikely to look after them, women are having fewer children. Once they are educated and employed, says Coleman, women, always the carers, find themselves under stress. "The culture is not conducive to the creation of new men, so women get overloaded and restrict their family sizes," he says.

Some interesting ideas about family culture and fertility there -- a subject on which much more could be said, including something about the extra-marital birth rate in Britain. For the moment, it is encouraging to see that demographic trends can change in quite a short time, and for the better.

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