High levels of the hormones progesterone and oestrogen are important for a healthy pregnancy but are often the cause of some common unpleasant side effects. Until the body has adapted to the higher levels of these hormones (which also includes HCG (Human Chorionic Gonadotrophin), Relaxin, Oxytocin and Prolactin), symptoms can be very common. Common symptoms include; 

- Mood swings  
- Nausea +/- vomiting (Sickness usually starts around week five to six and often subsides by week 12 to 16, although some experience it throughout pregnancy and/or have very severe forms of sickness). 
- Constipation   
- Urinary frequency  
- Incontinence  
- Tender, swollen breasts (breasts start to grow from weeks six to eight and continue to grow for the whole pregnancy so getting your bra size checked often is a great idea) 
- Fatigue  
- Food cravings or aversions  

Behind the scenes in the first trimester(1);  

- Resting heart rate begins to increase from weeks five to eight (you may well notice that during exercise your heart rate increases a lot more easily) 
- Blood pressure changes can be apparent from as early as week eight too (often there is a dip in blood pressure in the early weeks) 
- Your entire blood volume will increase by 30-40% by the end of pregnancy and this starts from weeks six to eight
- Your cardiac output increases by 50% by week eight and up to 75% by the end of the first trimester  
- Your red blood cell mass begins to increase between weeks eight to 10 (rising to 15-20% above non-pregnant levels by end of pregnancy) 
- Total cortisol levels increase at the end of first trimester 
 

For most, first trimester symptoms, if present, have begun or will soon be beginning to fade and settle as the placenta takes over hormone production and the body gets used to the levels. It’s during this trimester that often the gradual necessary weight gain will start to be more noticeable and most will start to see a bump during this trimester. For some, this can be the most physically enjoyable trimester of pregnancy.  

Most are advised to take iron supplements during pregnancy due to iron requirements increasing two to three fold as blood volume increases by so much(1) . One of the side effects of some iron supplements is an increase in constipation, which thanks to high progesterone, may already be a feature for some. Constipation and straining on the toilet increases our risk of pelvic floor dysfunction and haemorrhoids, so the more we can do to avoid this, the better. It’s worth noting here that not all iron supplements are made equally when it comes to effect on constipation, often you will find those in liquid form will not increase the risk but still provide the extra required iron. 

Behind the scenes in the second trimester(1): 

- Red blood cells continue to increase, reaching peak at week 28 
- Plasma volume continues to expand rapidly with a steady fall in haemoglobin throughout second trimester 
- Relative insulin resistance begins (to allow for energy availability for the foetus) 
- Increased pronation of the foot as pregnancy progresses 

Weight gain of your baby is most rapid in the third trimester, so this is when you will really start noticing a bump if you haven’t already. And the growing bump can start to have consequences; 

The resting position of the diaphragm (the main breathing muscles) shifts approximately 3-4cm upwards and the thorax broadens by 5-7 cm(1). Moderate exercise can decrease any feelings of breathlessness that may start to become apparent.  

Increased urinary frequency, urgency and incontinence – mainly now due to the foetal head down in the pelvis (be sure to be doing your daily pelvic floor exercises!) 

Along with the mechanical effects caused by the enlarging uterus, the elevated levels of progesterone contribute to slowing down gut transit times increasing the likelihood of constipation still 

Joint position changes occur due to increase in weight gain, increase in joint laxity, increase in low back curve and shift in centre of gravity(2) 

You may also notice that your feet change and have a knock-on effect to your footwear choices (reduced arch, increase in foot width and increase in water retention are all to thank for this!) 

You may also be noticing your relatively high heart rate. It increases by 10-20% (averaging 10-20 beats per minute higher) by 32 weeks.  

Sleep quality can decline as insomnia is more likely in pregnancy. Exercise helps negate this, along with getting comfy in bed. No such thing as too many pillows at this stage of pregnancy! 

Behind the scenes in third trimester(1, 2): 

- Circulating blood volume increases by 50% at full term 
- Increase in VO2 max of 20-30% by end of pregnancy – this can have an overall positive impact on exercise  
- Gradual and constant shifting of the abdominal organs to accommodate the rapidly expanding uterus 
- Risk of gingivitis (bleeding gums) increases, affecting 30-80% of those in the third trimester  
- Cortisol levels are three times that of non-pregnant values by the end of pregnancy  
- Insulin resistance peaks  
- Increased nutritional requirements – 300kcal more per day needed to maintain the metabolic demands of pregnancy