Science Questions

Upside down babies in the womb

Thu, 18th Jul 2013

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Marylou! asked:

Dear Naked Scientist.

I have a question that has bugged me for weeks and I hope you can shed some light on it.

I am approaching the end of my third trimester. The baby has got itself into the right position, which is great!! Though this is were my question lies.

A unborn baby's head points downwards in preparation for the birth and can be like that for many weeks. If I hung upside down for that long all my blood would be swimming around my brain and I would be a bit dead!! So how come a unborn baby is okay??? Thank you


Hannah - So, how does a baby withstand a prolonged head stand in the womb? To test the effects of gravity on an adult, we asked Claudia Efstathiou to do a headstand.

Claudia - Okay, so I'm just going to get in position now.

Hannah - Your face is starting to get a little bit red there.

Claudia - Yeah, I can definitely feel all the blood rushing to my head. It feels a little bit odd.

Hannah - A baby or foetus in the womb can be upside down for a month or so. Here’s Dr. Matthew Mason, Physiology lecturer at Cambridge University for his hypothesis.  

Matthew - The foetus is much smaller than an adult and it turns out that the pressure at the bottom of any column of liquid and that could be the blood supply is higher than the pressure at the top by an amount given by rho G H. Rho is density. G is the acceleration due to gravity and H is the height. The higher the height of the liquid column, the greater the pressure at the bottom relative to the top. Now, if you assume that an adult woman is let’s say, 160 cm tall, and if you imagine that a foetus is only 20 cm from top to bottom then that means that the adult is maybe 8 times higher than the foetus. That means that the pressure difference between the blood at your feet and the blood at your head is 8 times greater in the adult than it is in the foetus. That’s purely due to the

height difference that they're going to have a much smaller problem with being upside down than we are.

Hannah - So now, back to a fully grown Claudia to find out how she’s fairing.

Claudia - Alright! A little bit lightheaded. A lot of pressure on the top of my scalp doing that for a while.

Hannah - So, a small baby has a much lower pressure on its upside down head. As well as this, babies have amniotic fluid surrounding them. This external water pressure helps to balance out the pressure difference that we experience between our head and our feet in the outside world. And lastly, the body has sensors by the heart and also the head to regulate pressure, altering blood vessel diameters to compensate and act as a buffer as both adults and babies in the womb move around. Sticking with head, we next bite into this…

Steve - I'm Steve from Hampshire. I was in the car one day with my daughter and we were playing the, ‘see how long you can keep a sweet in your mouth without chewing it’ game. I gave up after a minute and started chewing my sweet and it got me thinking. What is worse for your teeth, sucking a sweet for ages until it goes to nothing or crunching it straight away and having that sugary goo stuck in all the nooks and crannies of your teeth, doing all the nasty stuff that sweets do?

Hannah - So, sucking sweets for a long time or crunching them to smithereens? Which one is best for your dental health? Let us know what you think. You can send us your thoughts to, you can tweet @nakedscientists, you can write on our Facebook page, or you can join in the live debate on our forum which is at


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The baby is suspended and feels weightless.  Equal pressure on all surfaces. There is no higher pressure in its head. The baby has no idea of up or down.  He or she has some surplus room but not much.  As the fetus grows and it gets more crowded and the head fits better in the lower, pear shaped pelvis and in routine turning, normally gets trapped, head down which is perfect.  The head shape even causes a posterior turn, again perfect. It cork-screws on the way out to face up, again perfect.  Pretty good engineering!  tecvia tecvia, Wed, 31st Jul 2013

The best engineer ever. Praise, Thu, 15th Sep 2016

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