When most people attempt HIIT training they are actually only doing 'MIIT'.
MIIT stands for MODERATE INTENSITY INTERVAL TRAINING and HIIT stands for HIGH INTENSITY INTERVAL TRAINING.
True HIIT training requires the activation of the anaerobic system of the body which means you need to be pushing it to the MAX during the 'time on' period. Recovery should then be as long as it takes for you feel like you can go 100% again.
Most peoples idea of a HIIT session is a series of 1 minute on/1 minute off intervals but in theory you would need A LOT of stamina to sustain 1 minute of max effort!
15-20 seconds is about as much as your body can take to really perform at 90-100%. It has to create the same metabolic response you get when you weight train heavy to failure.
Not ideal in pregnancy right?!
Pregnancy is not the time to be training at 90-100% max effort, even if you are used to that kind of intensity(which most people are not). Circuit or MIIT training where you stay around 60-70% effort is enough to feel like you've had a good workout. Remember its all relative- if you are a beginner, your 60-70% will be different to someone else"s who is used to training. Being able to maintain a conversation is a good indication that you are within range.
Exercising during pregnancy should be about staying strong, keeping good posture and feeling great, not about fat loss or endurance.
If you are trying to conceive, too much high intensity exercise can place additional stress on the adrenal glands which are a big part of conception. Cortisol is released by adrenal glands under conditions of high mental or physical stress and is the body's primary catabolic hormone.HIIT may stimulate cortisol response and that is something you want to keep at a minimum.
In my opinion, it is way more beneficial to include more lower to moderate intensity exercise such as light resistance training, yoga and pilates to relax and combat stress, recover quickly and reduce cortisol levels. Drinking adequate amounts of water, eating whole unprocessed foods and lots of sleep will help you to be consistent with your health and fitness goals without having to 'GO HARD' which is definitely not necessary when your energy is needed when growing a baby or trying to.