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Tuesday 10 September 2019

Weight (Things We Don’t Know about Pregnancy Series #2)

If you’re pregnant and you’ve been reading books, boards and information sites, like me, you’ve probably got the wrong end of the stick about some things – such as weight. And the reason is this: having a baby in the UK is very different from having a baby in the US, and the US dominates these platforms.

Partly, this is because the NHS is unforthcoming: they don’t share information about being pregnant until you are. Their websites offer the bare minimum; their leaflets aren’t much better; they don’t answer my emails; and their advice is always to talk to your midwife. As mine explained to me, information is drip-fed as you progress to “prevent you from being bombarded”. Or, in other words, to maintain control over information access.

Meanwhile, the US sites would lead you to believe that medical health professionals are obsessed with monitoring, managing, and manipulating your weight gain throughout pregnancy, including berating you if you gain too little weight or, more emphatically, if you gain too much.

Diagram of weight versus gestational age by Yehudamalul via Wikipedia Commons.

When I first saw a midwife, I refused to be weighed. I have generalised anxiety and I didn’t want the stress of obsessive monitoring, I told her.

She stared at me open-mouthed.

Monitoring a woman’s weight gain, she explained, is very “old school”, the amount of recommended weight gain is debated scientifically, and the process is counter-productive because it makes women worry. Perhaps they still do it in America, but they don’t do it here.

So what is the beef about weight?


Weighty matters, by Angelsharum via Wikipedia Commons.
There are two weighty factors: the mother’s starting weight and her weight gain during pregnancy.

When it comes to the latter, scientists really don’t know what is “best” or how far off-piste you can safely go without risks[1]. In fact, many women lose weight in the first trimester because of sickness, but go on to gain and have pregnancies and healthy babies.

Why do women need to gain weight during pregnancy anyway?


The weight gain during pregnancy goes towards several things: growing the placenta (which reaches a princely mass of 1.5 pounds), amniotic fluid around the baby (2 pounds), increased tissue masses (4 pounds), extra fluids (4 pounds), increased blood volume (4 pounds), and, of course, growing the baby (7.5 pounds). A further 7 pounds is stored nutrients, including fat, believed to be a vital resource during pregnancy and breastfeeding. Scientists don’t know exactly how this happens – only that it’s controlled by hormones. We know that hormones, including those involved in pregnancy, are linked with weight gain, loss and retention. Low levels of oestrogen, for example, may make the body less efficient at using nutrients and increase fat storage.

To achieve healthy pregnancy weight gain, UK sources recommend eating ~200 more calories a day, whereas US sources recommend eating ~300 more calories a day (mostly).

Because baby growth is affected by what you take in, when it comes to weight gain, the principal concern is healthy baby size. Very large changes in the mother’s weight produce changes in baby size, but this relationship is not precise – and moderate deviations are often not seen.

Very large or very small babies are at risk of more complications, especially small for gestational age (SGA) babies. SGA babies carry a risk of cardiovascular, respiratory and digestive complications, whereas SGA (large for gestational age) babies cause birth problems – usually leading to caesarean[2]. Or, in other words, small is bad for baby, big is bad for mother.

This makes SGA more dangerous, as baby is more vulnerable – so not gaining enough weight is more risky than gaining too much. In her book Expecting Better, Emily Ostler points out that, in this case, gaining weight so as to minimise the risk of either SGA or LGA babies isn’t necessarily ideal – as it weights the risk to baby and mother equally[2].

But weight gain during the pregnancy is not the only thing that affects baby weight. Even more important is when the baby is born – early, late, or close to their due date. Needless to say, babies get bigger with time, so comparing mother’s weight gain for early and late babies doesn’t give useful data. There has been a suggestion that gaining weight too fast or too slowly can cause a baby to be born early (or just too slowly – scientists disagree)[3]. Baby size also depends on the mother: any conditions she may get like gestational diabetes, and her starting weight.

Babies get bigger as time goes on, via Pixabay.
It turned out my midwife wanted to weigh me to check whether my starting BMI was within the “normal” range. Over- and underweight women can be at increased risks of pregnancy complications. However, overweight and obese women are still recommended to gain weight during pregnancy – just less than women of a healthy weight, reducing the amount of stored fats.

Being obese carries a chance of reduced fertility, higher incidents of miscarriage, stillbirth, birth defects, gestational diabetes, preeclampsia, sleep apnea, blot clots, and a tricky delivery/recovery. It can also complicate spotting birth defects. Being underweight carries the risk of premature birth and SGA at birth – and so risk of cardiovascular, respiratory and digestive complications in the newborn. However, these are risks: babies born to over- and underweight mothers can be perfectly healthy, and babies can have complications born to mothers of an average weight. Other factors are involved, and scientists can’t identify all of them.

Some have suggested that heavier mother or fast weight-gaining mothers (often the same group) may cause their children to grow up to have higher BMIs[4][5]. If so, the mechanism behind this could be insulin resistance. However, it’s impossible to entangle weight factors from genetic and environmental factors such as family eating habits. Other research has suggested that in some cases increased BMI can skip a generation, or is in fact linked to the microbiome[6].

Caesarian babies don't necessarily get their microbiome from their mothers by Beth [CC BY 2.0], via Flickr Creative commons.
So what about me? I did let the midwives weigh me once – when I went for my 12 week dating scan. Amongst other things they perform various blood tests, including screening for STIs and, in case they need to treat you, calculate the correct drug dose based on your weight. However, I asked them not to give me the number!

Another concern of many women is losing weight after pregnancy. Under pressure from celebrities and internet fads, there is an expectation to do this as quickly as possible. A healthy weight loss rate would get most women back to pre-pregnancy weight in 2-3 months, but in practice most women have only shed 86% of their baby weight by 6 months[7]. It’s not medically clear whether losing this weight right away is healthy: that weight is partially gained to support breastfeeding, and we know a huge number of hormones and antibodies are passed between mother and child this way. Research is ongoing.

So yes, there are many unknowns when it comes to pregnancy, and over the next few months, I’ll be exploring more of them with you. Look out for my next blog post, which will be about bizarre symptoms.

To read our full article on the things we don't know about pregnancy, check out our site. This article will be updated as we add posts across the coming months.

why don't all references have links?

[1] Abrams, Barbara, Sarah L. Altman, and Kate E. Pickett. Pregnancy weight gain: still controversial. The American journal of clinical nutrition 71.5 (2000): 1233S-1241S.
[2] Emily Ostler, Expecting Better: Why the Conventional Pregnancy Wisdom is Wrong - and What You Really Need to Know (2013) Penguin Press.
[3] Bodnar, Lisa M., et al. Severe obesity, gestational weight gain, and adverse birth outcomes. The American journal of clinical nutrition 91.6 (2010): 1642-1648.
[4] Fraser, Abigail, et al. Association of maternal weight gain in pregnancy with offspring obesity and metabolic and vascular traits in childhood. Circulation 121.23 (2010): 2557.
[5] Reynolds, R. M., et al. Maternal BMI, parity, and pregnancy weight gain: influences on offspring adiposity in young adulthood. The Journal of Clinical Endocrinology & Metabolism 95.12 (2010): 5365-5369.
[6] King V, Dakin RS, Liu L, et al. Maternal obesity has little effect on the immediate offspring but impacts on the next generation. Endocrinology. 2013.
[7] Antonakou, A., et al. Role of exclusive breastfeeding in energy balance and weight loss during the first six months postpartum. Clinical and experimental obstetrics & gynecology 40.4 (2013): 485-488.

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