There do seem to be some principles if you can call that that are more like suggestions gained through experience.
Relative duration and actual duration can be lengthened over time as this has been an observed tendency over time with yoga, and progressively.Learning is best done incrementally and applied.So one could start off with 1-1, make the observations, then proceed accordingly.Like bio-feedback training.Also using common sense.Lentgtheniing exhalation can be introduced gradually.And retention is not something, if i was a teacher, i would (invariably) introduce in the first instance straight out the gate because as a stimulator it could be too powerful …But that is just me.There plenty of ways to potentize pranayama.I personally fel pranayam is powerful and effective enough without adding internal kumbhaka(retention after inhalation) what is usually prescribed first,.If pauses ooccur naturally that is different.You could draw a comparison with learning pranayam to playing a musical instrument.It demands concentration, finding & establihshinig a rthytm,devloping receptiviity and awareness to the sounds of the air and more significantly the prana once a subtle awareness has been cultivated.Some are less aware,receptive or of varying states of health so other variables can be factored in, and so on. Like anything in life when you’re learning there are the experience which reveal signs as to what is apporpriate for any given individial at any given time and place.How it fits into any given over-all practice.What separates breathing from breathing with awareness and focus.Remeber that you’'re coaxing the nervoussystem open gradually gentlly. So you treat the breath like a baby.; it teaches you and you can guide and teach it. it will let you know when you’re on it’s wavelength. There are various tips and tricks that can potentize pranayama that is wsest to learn incrementally and progressively. That is not to fear the breath,certainly not.But when you rock a baby you rock it gently first and then you pick up the cues.
I personally don’t employ kumbhaka that often(unless as i am consciously going up a gear deliberately) as my pranayam is powerful enough (so why make it more so when it dooes’nt have to be)However it is not appropriate to say that is not appropariate for you.Only a practioner or a practitioner getting guidance or imbibng teachings in conjunction with the company and guidance a teacher can be a reasonable judge of that.
There are common-sense principles that can be respected & employed but there are’nt really any universal prescriptions other than the suggesstion to begin with basics.I know many can feel scared by pranayama,and they should’nt be if they feel they would benefit from it., but youwould’nt be best to drive a formular-one racing car with some tuition or knowledge about the controls and how they might affect the vehicle’s behaviour.The main thing is experience is gained through learning and application which accrues over time.A useful practice for one individual may not be best for another.And pregnancy is one variable amongst many.If you can maintain reasonable focus and awareness for extended periods of time while observing subtle cues,the effects of the movmenet of prana throughout the so-called “subtle” body, which you can recognise as tell-tale signs then that’s part of the learning.
There are many tips & tricks in pranayama. A good mind-set, prepration because it is like a half-wy meditative state before deep meditation. If one is sill agitated after asana then you can always do your asana affter or spend longer in savasana.How does the diapghragm appear to respond and so on.The breath is best quiet-ish in most pranayama.It would be difficult to offer any unviersal prescriptions to every individual other than 1:1 leading to 1:2 in increments. What may be observed also and while this is being done is that the actuall duration lengthens as well.Maake anote of these observations,whether with a count or without, whether the breath sounds a certain way or does’nt,how you feel physically,mentally,greater clarity,the third-eye,the crown perhaps, the body etc and proceed accordingly.When you’ve finished a couple o basic cycles sit and observe any discernible results. Discriminate when the breath might appears to be struggling or is flowing naturally and easily,unobstructed,without resistance and so on…