The ethics of having (more) children

Laurie Paul wrote an excellent paper, a few months back, addressing the epistemology of choosing to become a parent. The ethical question, of whether or not to choose to burden the planet with one's progeny, is one that has received plenty of attention since contraception became widely available in some countries in the 1960s. But a related topic, or perhaps sub-topic, that I don't believed has been addressed in an academic setting, is the ethics of choosing to get pregnant when one already has one or more children. In particular, I'm wondering about the moral issues connected with subjecting one's child to siblings.
I suspect the ethics of reproduction make an excellent test case for ethics more generally, because moral facts, if there are any such things, are generally taken to be closely connected to unalterable facts about human life. Moral intuitions seem to be strongest, at least, when they concern events or processes which are universal in the human species, and which concern the more animal, biological aspects of our existence. Nothing could be more animal than begetting offspring (trust me) and for 99.999999 etc % of the time in which the human race has been gestating and refining its moral sentiments, pregnancy and parenthood have been things over which we have had little real control. So it seems plausible that most of our moral intuitions concerning the ushering in of new life are fit, if at all, for an outdated mode of existence. Having children is something a majority of adults still seem to just take for granted that they will do, at some point in their lives. But now that they don't need to take it for granted, now that it is not inevitable, are our intuitions and expectations going to gradually shift away from this norm? And, assuming that ethical facts are not identical with these norms, isn't it ethics' job to lead the way?

There are certain facts that are relevant to the decision to have a second child that are not attached to the decision to have a first child. Namely, the rights of the first child. For example, any time a couple decide to try to get pregnant, they face the possibility of the child being born disabled or sick.  They must weigh this chance, and the resultant pain or unhappiness it might burden the child with, up against the chance of having instead a healthy happy child. But those who are parents already must additionally factor in the extent to which the unhappy outcome would adversely affect their first child.  There will be those who claim that having a disabled or differently abled sibling can be a huge advantage and source of joy rather than a cost, but the possibility remains that this is not how things will turn out.

Of course, many things in life are risky but worth doing anyway. But isn't there something uncomfortable about the fact that children get no say at all in these matters that, however they turn out, are going to turn their entire world upside down? Isn't there some validity to a child's perspective which says 'I like my life just fine, I like being the centre of my parent's attention and having access to 100% of their resources, and while its true that I might enjoy having a sibling later on, I don't really want to take my chances on all the horrible ways this could turn out. My parents say they really love me, but I cant help feeling that if that were true, then I would be enough for them, they wouldn't need to have another throw of the genetic dice to see if daddy's violin skills and mummy's way with watercolours can combine to produce a little genius next time........'

David Haig has recently argued that babies are evolutionarily hard wired to produce behaviours, especially crying at night, and breastfeeding at regular intervals even when they don't need to eat so often, in order to prevent their parents from conceiving subsequent children. And recent research in the UK suggests that a staggering percentage of Brits report being bullied at some stage in their childhood by a sibling. Yet somehow parents often seem to convince themselves that further procreation is actually in the interests of the existing children.

People make up all sorts of excuses to justify the act of having a second (or third etc) child, including;

• It's to give the first child a companion. 
• It's because they have some left-over 'love to give'.
• It's because the first child is so fantastic they don't want to squander the opportunity to give the earth a second, equally fantastic, person. 
• It's to prevent the first child from becoming spoilt.

Most parents probably assemble all of these at once. But are these just post-hoc justifications in response to a biological inevitability that no longer obtains? Is the conceiving of child n+1 ever morally indefensible?