Simplified Science: Pregnancy and your body

I think that this is rapidly becoming one of my favourite series on AOW. Katie has a way with words that can’t be beaten. Not only does she have a talent for making super complicated things sound super simple, but she does it with hilarity and humilty. 

I specifically requested this topic, because I think that knowing WHY my body was doing what it was doing, might have made it easier to endure the nausea and the swollen ankles that were pregnancy for me. I didn’t realise at the time though, how many of you, including Aisling, mother-to-be-of-Stella-Temperance, would be announcing your pregnancies, and how relevant this might be for some of you right now. You’re welcome.

You all know by now that I think it’s both hugely important and exciting to learn and understand how your body works, and one of the most amazing things your body can do, if you’re a woman, is to grow a baby. A whole actual new person. Without going into any details, that’s already pretty freaking incredible.

Pregnancy though is not without its challenges, and the ability to grow a person has a cost, from morning sickness to constipation, via approximately 10 million trips to the loo and a spectacular range of mood swings. So what can you thank for these side effects? Your hormones. Your hormones orchestrate your entire pregnancy, all the good bits and bad bits, in fact, from the very moment you conceive, your endocrine system is performing a ballet of efficiency that would have clipboard-wielding bureaucrats the world over weeping in envy. Unfortunately, the swooping levels of hormones that are required for the Ballet of Growing Feet upset the usual precise balance of your endocrine system, leading to sudden changes in mood.

This is going to be a whistle-stop tour of pregnancy hormones, as always it’s a huge topic and it cannot be covered in detail in one blog post, unless you’re all prepared to sit and spend your whole day reading this one post (I’d totally provide cake if you were). This is simply an insight into how your amazing hormones orchestrate some of the incredible and some of the less delightful aspects of pregnancy. And as always, please remember that I’m a massive science nerd, not a medical professional. I think it’s amazing to understand how your body works and how your hormones can cause side effects, but if you are suffering severely from any pregnancy side effects, you absolutely need to contact your GP and get proper medical treatment.

So, in my previous post on the pill I described how hormones orchestrate your menstrual cycle, up to the point of egg production. Once the egg is produced, however, the follicle that released it doesn’t get to lie down after all that hard work, but instead has a midlife identity crisis, changes its name to corpus luteum and gets to work pumping out progesterone. The egg meanwhile shimmies on down to the fallopian tube, there to linger enticingly in the hope of snagging itself a sperm-buddy. I’ve never been able to help picturing the egg at this point as a Marilyn Monroe-esque babe cooing “come and get me boys” down the fallopian tubes to the sperm. In fact, cells surrounding the egg release progesterone that ‘draws’ the sperm toward the correct place, and once the sperm is nearby, the egg is able to capture a sperm swimming nearby using sticky sugar molecules on its surface. And once your seductive egg does capture a sperm, the game is officially on.

In terms of hormones, human chorionic gonadotropin (hCG) is first off the block. As soon as the cells of the placenta start to develop, they start to produce hCG. hCG in turn heads back to the corpus luteum, to encourage it to produce huge amounts of estrogen and progesterone. The levels of hCG in your blood and urine at this stage are what indicate whether or not you’re pregnant, when you pee on a stick, hCG is what you’re looking for.  The levels of hCG remain relatively high for the first few months of pregnancy, the levels start to decrease as soon as the placenta is capable of taking care of progesterone and estrogen production, allowing the corpus luteum to retire gracefully to a holiday home by the sea. During this time, as well as helpfully encouraging the post-midlife crisis corpus luetum into estrogen and progesterone production, hCG is also having some other effects, most importantly for many women, it is the prime hormonal suspect responsible for morning sickness. Since there is no evidence to explain how hCG would cause morning sickness, there’s no guarantee that hCG is to blame; however there is a very strong correlation between the rise and fall in hCG levels and the typical timing of morning sickness. Lovely.

As well as potentially being responsible for morning sickness, hCG also has some other side effects. First, it acts to suppress your immune system, making you potentially more susceptible to infection. At first this seems like a really dumb thing to be doing, but actually it’s really important that your immune system doesn’t get too over-excited, recognise the baby as a foreign object, and act to destroy it. So if you are suffering from a cold in the first few months of pregnancy, remember that it’s in a good cause. The final effects of hCG is to increase blood flow to your kidneys and pelvic area, in order to help them become more efficient. Obviously this is important because your kidneys are soon to be processing the extra waste produced by your baby, and they need to be able to cope with that increase. The downside? All that increased blood flow down there is what sends you to and from the loo every half hour.


hCG is not the only pregnancy hormone, however, and as I said, its big job is to help produce the two hormonal headline acts of pregnancy, oestrogen and progesterone. The main role for oestrogen is to act somewhat like the foreman of a building site on speed; it regulates the activity of other hormones, and zooms around the body, encouraging the lining of the uterus and breast tissue to grow. It’s also inside your baby, orchestrating the growth of baby organs and limbs, particularly regulation of bone density. As part of its role as the foreman of pregnancy, oestrogen also increases blood flow to various parts of the body, including your breasts (to help them grow), your skin (hello flushed-face pregnancy glow) and your palms, which is why they can become sensitive and itchy. Needless to say, by the time you give birth, your oestrogen (having helped your grow feet and all that jazz) is probably worn out, and may need a lie down somewhere quiet.

Progesterone is the second of the celebrities of the pregnancy hormone world, and amongst other things it is the official hormonal placental cheerleader, keeping the placenta functioning properly throughout its long 9 months of work. Progesterone also acts along with the hormone relaxin to create much-needed room in the uterus for the growing baby by relaxing the smooth muscle in the walls, allowing them to stretch out. Clearly very important, no? The problem with this is that it also affects the normal function of smooth muscle in the intestines, leading to a delightful range of side effects from indigestion to flatulence to constipation.

Finally, progesterone has a key role in controlling any contractions of smooth muscle in the walls of the placenta, right up until that crucial moment…




Categories: Becoming a Mother, Health
32 interesting thoughts on this


  1. Katielase
    Posted September 19, 2012 at 7:11 am | Permalink

    Oh! It’s me! Again. Should REALLY start trying to remember the dates my posts are due to go up….

    I’m not going to be around online much today, but if anyone has any questions after reading that, I promise I will come back and try and answer!

    K x

  2. Vivienne
    Posted September 19, 2012 at 7:12 am | Permalink

    Cake? Did someone mention cake?

    Pregnancy is pretty amazing. My mum laughed at me after our 4D scan when I told her ‘I know I’ve grown a whole baby, but I’ve made toes and fingers and a willy!’ Which my hormone addled brain finds even more awe inspiring. I’m laying in bed with those very fingers and toes poking away at me to get up and feed him breakfast and to imagine he once was a combination of a few cells brought together over a bottle of v nice red wine and posh steak just before Valentines Day.

    Another fabby science post – please do keep them coming!

    • Carly
      Posted September 19, 2012 at 7:28 am | Permalink

      ‘I’ve made fingers, toes and a willy!’

      Definitely my quote of the day – had me laughing out loud!

      Great post Katie, I’ll file it away to read again when the time is nigh.


      • Steff
        Posted September 19, 2012 at 9:33 am | Permalink

        Me too, actually hilarious! x

    • Liz
      Posted September 19, 2012 at 9:45 am | Permalink

      Vivienne, think I am due just about the same time as you (31st Oct for me!). Also finding it pretty magical and incomprehensible when I properly start thinking about it! The bit that I really go wow about is that this baby already has hair and eyelashes and fingernails, and has done for quite a while now!

      katie, another really interesting post!

    • Martha
      Posted September 19, 2012 at 1:50 pm | Permalink

      Haha, this made me giggle. My friend is terrified of having a baby as she finds the concept of “growing a willy” far too disturbing!

    • Katielase
      Posted September 21, 2012 at 10:04 am | Permalink

      Ha! Amazing. You guys are all so impressive, you’ve all grown feet and eyelashes and fingernails and hearts and lungs and legs and just W.O.W. Freaking incredible.

      K x

  3. Anon
    Posted September 19, 2012 at 7:52 am | Permalink

    Oh Katie, I love you for writing this post right now! I’m currently 11 weeks pregnant (hoping to come out of the AOW pregnancy cupboard next week) and have been amazed and fascinated by my body and all it is doing to make this baby. For me, pregnancy kicked off with a cold (followed quickly by all day morning sickness), and I think it’s just so clever that that’s essentially in order to keep the baby safe. Morning sickness and tiredness has been pretty all consuming for the last however many weeks, but I keep reminding myself that I’m growing a brain (bloody hell!) and that makes it a little easier to bear.
    More like this, please Katie!

    • Steff
      Posted September 19, 2012 at 9:31 am | Permalink

      Ooooh another one joining the club :) exciting times!! Lucy will be doing her nut waiting to find out who you are though!! Hehe xx

      • Posted September 20, 2012 at 10:52 am | Permalink

        Yes, yes I am! And just when I’d recovered from the relief I felt at finding out you were the last secret AOW baby maker ;)

    • Posted September 19, 2012 at 10:35 am | Permalink

      You make it sound like Anna and Clare are keeping Aisling in a cupboard!

      Can’t wait to see who you are!


      • Posted September 19, 2012 at 10:38 am | Permalink

        That’s EXACTLY how it happened, Esme. I was like Harry Potter, but fatter.

      • Posted September 19, 2012 at 11:07 am | Permalink

        Esme, I think you misunderstand. We ARE keeping Aisling in a cupboard. She can’t be trusted in public. She wears Bambi jumpers.

        • Posted September 19, 2012 at 11:14 am | Permalink


          And it’s only ONE Bambi jumper. For now.

    • Katielase
      Posted September 21, 2012 at 10:06 am | Permalink

      Eeeep! Eagerly waiting your emergence from the AOW Cupboard of Pregnant Ladies. Are Bambi jumpers compulsory clothing in there?!

      Glad this helped a bit, every time you feel worn out by pregnancy, remember that you are growing a whole actual human. You cannot underestimate how amazing that is, and how much effort your phenomenal body is putting in to do that.

      K x

  4. Posted September 19, 2012 at 7:59 am | Permalink

    Clare— are you having another baby??

    I seriously love the science series- good work Katie.

    Also, if you love these posts too make sure you get a look at your placenta after delivery! It’s completely gross but also completely fascinating. Don’t tell George but I was looking at that longer than him in the first ten minutes.

  5. Posted September 19, 2012 at 8:00 am | Permalink

    OK I take back the question after reading the intro’ four more times. It’s definitely past tense.

    • Clare
      Posted September 19, 2012 at 12:44 pm | Permalink

      Haha – no Anna, that was not the sneakiest AOW pregnancy announcement ever!

  6. Rach
    Posted September 19, 2012 at 8:10 am | Permalink

    I marvel every single day at the fact that there is an actual human growing inside me. Another heart, bones, a brain etc. Totally mind-boggling. Another brilliant post, my favourite series on AOW. Thanks Katie! x

  7. Vivienne
    Posted September 19, 2012 at 8:11 am | Permalink

    Ooh, another AOW baby to join the club, will await the ‘reveal’ with great anticipation Anon!

  8. Posted September 19, 2012 at 8:14 am | Permalink

    Hope all is well at your scan anon! X

  9. Steff
    Posted September 19, 2012 at 9:35 am | Permalink

    Katie I straight up love you right now. This is bloody brilliant – sending to all my pregnant friends as we speak. xx

  10. Posted September 19, 2012 at 10:35 am | Permalink

    So interesting! Thanks, Katie!


  11. Posted September 19, 2012 at 10:37 am | Permalink

    Katie, you’re so awesome. Such a geeky genius – I love you!

    I always made sure I read up on what my body was doing (or not doing, as was the case…), I found that understanding WHY I was infertile made a huge difference to how I felt and cope, especially early on. Phil and I read and researched the drugs and operations I had like crazy – being able to talk about them like we had half a clue helped hugely. We’ve approached pregnancy the same way – C is bang on in her intro, I think understanding why your body is behaving so strangely is a massive part of accepting the changes and not fighting against them.

    I did mention that I love you, didn’t I Katie?!

    A x

    • Katielase
      Posted September 21, 2012 at 10:08 am | Permalink

      I love you too. There, I said it. Phew.

      I’m exactly the same, I always need to know what’s going on with my body and what the drugs and treatments do, it helps me feel a bit more in control of myself, like it’s not just a runaway train that my body is on, and I’m watching helplessly from the sides. You are Clare are spot on when you say that understanding often leads to increased acceptance.

      K x

  12. Frances
    Posted September 19, 2012 at 2:01 pm | Permalink

    Fascinating stuff – thank you!

  13. Posted September 19, 2012 at 3:35 pm | Permalink

    Katie’s science series is amazing!! The ‘what the pill does’ post pretty much changed my life, so hopefully if and when I ever decide to have children and freak out a bit, I can dig this post out, read Katie’s wisdom and remember that it is incredible and not something evil making me fat. No, that was the pill.

    L x

    • Katielase
      Posted September 21, 2012 at 10:09 am | Permalink

      Laura, your response to the pill post remains the best thing that has ever happened to me as a science writer :-)

      K x

  14. Katie
    Posted September 20, 2012 at 10:25 am | Permalink

    Catching up with my AOW reading, and this is brilliant. I’ve got books on what my body’s doing, week by week, but Katie, you put it so much more succinctly, I love your science posts. They are easy to understand, and don’t take too much concentration (can you tell I’m a dippy blonde?).


  15. Katielase
    Posted September 21, 2012 at 10:11 am | Permalink

    Thank you all, as always for the responses. I didn’t realise when I wrote this how many AOW-ettes would have announced their big news by the time it got published, but I’m glad it’s been appropriately timed! Next up, giving birth… your hormones do the hard work so you don’t… oh, wait. No, Okay, that needs work…

    K x

2 Trackbacks

  • By Behind Closed Doors: IVF – The Lowdown on November 12, 2012 at 7:00 am

    [...] different.  Nor is it about the science, as I don’t really understand that (read this post to understand what IVF is doing that your body otherwise naturally does.) This is a post about the actual, the practical. I deliberately kept away from any conception [...]

  • By Self-esteem and mortgage advisors on December 17, 2012 at 7:02 am

    [...] We’re kicking off with a belter today from Katie, aka Katielase, she of the brilliant Simplified Science posts, she who I met last week and entirely forgotten I’d never met before as she’s so [...]

Post a Comment

Your email is never published nor shared. Required fields are marked *


You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>


Hello! We're Clare, Aisling and Anna and welcome to a corner of the world where smart, flawed, real women talk about the bigger picture; about their experiences, stories and opinions on all aspects of being a woman today, from marriage to feminism to pretty, too.

More here.

image by Lucy Stendall Photography

Find me a random post