Pain during labour is certainly not a manifestation of the fear-tension-pain syndrome. I am not sure what you mean by relaxation techniques. According to studies which are evidence based, the only way these classes really help is by education. The more you know about childbirth the better you may be able to cope with labour and make informed choices about how you want to go through labour (e.g. about the choice of anaesthesia). If these give misinformation or make you less confident in handling labour they are obviously not doing their job. Another reason you may go to these gatherings is that you meet other expectant couples in these classes which may be good thing if you share experiences and anxieties and see others in the same boat with similar anxieties and fears. This may, however, be counterproductive if you scare each other more than help each other. The pain in labour is very real and not imagined and needs proper management. To call it a manifestation of fear or whatever is not only medically incorrect but also unkind and patronising.
There are various ways of dealing with it from Epidural Anaesthesia on one end of the spectrum to exercises and alternative methods like faith healing and accupressure on the other. All these methods of pain relief have their relative advantages and disadvantages. You need to learn about each and make informed choices (like everything else in life). These methods have various degrees of success and side effects. For example, epidural anaesthesia or controlled spinal anaesthesia may alleviate pain more effectively but may also increase your chances of instrumental delivery (Forceps and Vacuum) and caesarean section with all their disadvantages and complications. On the other hand, methods like controlled breathing and accupressure may be less effective but are relatively harmless.
There is a lot of unjustified anxiety associated with pregnancy in what is called a fear of pregnancy which seems to be becoming increasingly prevalent in our society especially over the last twenty years or so. This is a direct aftermath of what could be called an over-medicalised management of childbirth. Somehow, most pregnant women these days seem to believe they are carrying some potentially dangerous condition. There is no denying the fact that normal pregnancy may lead to unanticipated complications (here I must hasten to add that majority of pregnancies are normal and happily end in a healthy mother and child) and medical help should be sought and indeed provided as and when such complications occur. While it is desirable to get some medical supervision and care during pregnancy, this should be to provide education and not increase fear.
To predict preterm labour/large or small by fetal weight estimation at this stage or of indeed any other outcome on the basis of an ultrasound examination is medically incorrect and makes no sense to me at all. You have also written about diabetes being ruled out. You are probably being told that you may be carrying a large baby. Please get this clarified by your treating physician. If he/she is suspecting some complication it should be stated clearly. If you are not having any problems your pregnancy should be allowed to continue normally without so much anxiety. Management of your pregnancy cannot be based on estimates and fears.
Please read about pregnancy on this website www.doctorndtv.com or www.rcog.org.uk care of the healthy pregnant woman (NICE i.e. National
Institute of Clinical Excellence guidelines). Also, for management of small for dates babies (I know you say your baby may be large but this page will tell you about the relevance and irrelevance of ultrasound estimation of fetal weight at 26/28 weeks). Also refer to Effective Care in Pregnancy and Childbirth by Murray Enkins (the book is free online or available at Oxford medical for a few hundred rupees. It is available at www.freemedicalbooks.com or just go to WebMD / yahoo health (pregnancy).
I hope you are able to educate yourself and fear the pregnancy less. You owe it to yourself. I hope this give you more confidence and helps you to enjoy your pregnancy. This is the time to knit booties and shop for the little one and not get paranoid about what all can go wrong in a pregnancy. Let your treating doctor worry about all that and while the pregnancy is going along fine please do not interfere with nature or get too many tests (including the so called tests of fetal well being like Ultrasound and Manning score or NST). Please have faith in nature and its systems and allow the pregnancy to progress naturally. Medical help is required when these systems fail and not as a replacement for natural childbirth. I hope you enjoy the rest of your pregnancy and the motherhood thereafter. I wish you all the best.