So the Duchess of Cambridge is pregnant again. The speculation about a brother or sister for Prince George began within months of the media frenzy that surrounded his birth. If the crowds outside St Mary's Hospital in London in July last year are anything to go by, it's not likely to end any time soon.
The announcement of the Duchess's first pregnancy was made earlier than usual, before the end of the first three months (the first trimester), when she was admitted to hospital with hyperemesis gravidarum - a very severe form of morning sickness which causes dehydration. Unfortunately for her, it seems she has not escaped it second time around, either - this pregnancy, too, was apparently announced before the 12-week stage for the same reason. Last time around, there was much speculation on whether she was expecting twins - a known risk factor for hyperemesis. This time everyone knew she was at higher risk - hyperemesis in one pregnancy increases the risk of getting it in the next.
But what about the timing of her pregnancy? She was 31 when she had her first child and is bound to know that fertility drops after the age of 35. If she wants three children, as she is said to, she may have felt pressure to 'crack on' with her next pregnancy. But she and Prince William almost certainly went through the discussion thousands of UK couples have every day - how long to wait?
It's unlikely that job prospects or money problems featured in the Duchess of Cambridge's list of issues to consider, as they do for so many others. But the health of her future baby almost certainly did. The World Health Organization (WHO) recommends a gap of at least two years from giving birth to getting pregnant again (1). However, the WHO provides recommendations for the whole world - and the time you need to get back into good shape after a baby in famine-affected Africa is likely to be very different to the time in Windsor.
It is surprisingly difficult to tease out the risks of second pregnancies after different lengths of time because of a host of 'confounding factors' - in other words, every woman is different so it's hard to generalise. However, there is some evidence that leaving a very long or a very short gap between pregnancies can take its toll on both mother and baby. Pregnancy takes a lot out of you, literally and metaphorically. While you may not be 'eating for two', your baby will drain your reserves of vitamins and minerals, and the demands continue if you breast-feed. The hormone relaxin, which may help to loosen your ligaments and make labour easier, makes you prone to strains and sprains. Your pelvic floor and abdominal muscles are stretched and weakened. And that's before we've even started on the sleep you need to catch up on after your first few months as a mother.
There may be risks for the baby too. Leaving a gap of under 18 months (but particularly under six months) between delivering one baby and getting pregnant with the next increases the risk of premature birth, low birth weight and having a baby which is small for its level of development. (2) There also seems to be an increased risk of autism in babies born very soon after their older siblings. (3)
But leaving too long a gap carries risks as well. With a gap over five years, the risks of pre-eclampsia, as well as problems in labour, go up.
So is just under two years, as the gap between Kate and William's first and second born seems likely to be, about right? They certainly seem happy about it, and that's what matters.
2) Conde-Agudelo A, Rosas-Bermúdez A, Kafury-Goeta AC. Birth spacing and risk of adverse perinatal outcomes: a meta-analysis.JAMA. 2006;295(15):1809.
3) Cheslack-Postava K et al. Pediatrics 2011; 127 (2): 246-253
4) Conde-Agudelo A, Rosas-Bermúdez A, Kafury-Goeta AC. Effects of birth spacing on maternal health: a systematic review.Am J Obstet Gynecol. 2007;196(4):297
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