In the UK, 22% of women don't have children. In Europe, this rises to 40% amongst graduates. Despite this there are around 700,000 births each year and on top of this a much smaller 200,000 net immigrants to bolster the population.
To be consistent, those arguing for women to have more children on economic grounds, should argue in favour of immigration even more strongly. Immigrants are generally fairly young with 30 to 40 working age years left in them, with useful skills that our economy needs and most importantly have not been a drain on the economy for maybe 16 years of costly education.
The fact that a lot of people argue against immigration AND at the same time for increased births can I suspect, only be for reasons of xenophobia and racism! As AC Grayling puts it (you may notice recently I'm currently quoting a lot from a few of his books I'm reading at the moment - and excellent they are).
"Nationalism (along with racism, religion, revenge, poverty and depression) is an evil. It causes wars, it roots lie in xenophobia and racism, it is a recent phenomenon - an invention of the last few centuries - which has been of immense service to demagogues and tyrants but to no-one else. Disguised as patriotism and love of one's country, it trades on the unreason of mass psychology to make a variety of horrors seem acceptable, even honourable."
There are plenty of people in the world, in fact far too many for the planet to sustainably support (The Catholics and other religions have done untold damage in this area, by discouraging contraception). Europe can easily import the millions of twenty and thirty something workers it needs. Women who don't have children should be given a medal. I am actually very surprised that 78% of British women do have children. Children are very expensive both economically and emotionally. The more educated women are the more they increasingly decide it is not worth it and I don't blame them, they are helping themselves and the world immensely.
Women in the modern age are faced with the impossible conundrum, for career prospects they are best delaying children to after 30, but this drastically reduces their chances of having them. In biological terms, women are probably best having children as teenagers, but economically this is disastrous. The less children women have, the more we can divert resources to those who do have children to improve their education and life chances. A fairer, more focussed and egalitarian system of education is the best way to make progress in eradicating poverty and reducing our skills shortage and it is a scandal we don't do this. (Of course, the growing influence of religion doesn't help with it's irrational arguments against evidence based learning and morality).
The one legitimate accusation that can be levelled against large scale immigration from the developing world is that we are robbing these poor countries of skilled workers they can ill afford to lose. This is why we shouldn't discriminate in favour of skilled workers, but take all skill levels. The remittances these workers send back to their countries of origin can be an essential income for the poorest parts of the world.
The global system is built for the capitalists, the Victorian slave labour of yesteryear has largely been relocated to Asia. But time will run out for the capitalists and ultimately all of us in the West who benefit, as labour there becomes more organised. At least one can hope that will be the case. In the meantime our democratic accountability is limited by the lack of democracy in countries such as the US, which sets tax rates far too low to support decent welfare (or spends it instead on the military industrial complex) and cheat by social dumping - stealing skilled workers from countries that do invest in their people. Not only is this unfair, it is inefficient to have this transfer of workers.
So to summarise, lets have much less children and slightly more immigrants, at least until the world economy is more equalised and world population growth more stable and this transfer of people is no longer needed. Contrary to popular belief, densities of population are set to fall (in comparable terms) in the UK compared to the rest of the world and our growing prosperous economy is more than capable of absorbing more immigrants, especially if we reduce the number of births. Even the more outrageous predictions of population growth only predict a rise of 9 million by 2050. Of course (like the weather), it is impossible to predict this far ahead, and lower births would easily reduce this rate.