the causes are manifold : their weight 
 increase on the one hand , and the specific physiology of the spine on the other . during 
 pregnancy ,
a woman s body weight increases by 15 to 25 percent ; this signifies a greater 
 burden on the tendons , ligaments , and joints1 .
furthermore , relaxin and estrogen loosen the ligaments and thus 
 create an additional predisposition for injury2 , 3 .
the enlarged uterus and the increase in 
 breast volume shift the body s center of gravity to the front .
the pelvis is tilted 
 simultaneously , and lumbar lordosis increases1 . in the course of evolution
, the female body has reacted to the special circumstances of 
 pregnancy by adapting the structures of the spine .
thus , the lumbar spine is more 
 hyperlordotic in females than it is in males .
furthermore , three of the lower lumbar 
 vertebrae are interlocked in females ( as opposed to two in males ) , and therefore are 
 stronger than those in males .
thus , shear forces between the vertebrae are reduced , and they are able to move further 
 backward to balance the weight increase in the frontal aspect without damaging the 
 spine4 . the aim of the present study was to determine the time of onset of back pain during 
 pregnancy , and the accompanying changes in posture with respect to the kyphosis angle , the 
 lordosis angle , and trunk inclination .
data concerning posture analysis were processed with 
 the aid of a biomechanical model developed by the authors , in order to draw conclusions 
 about bending moments and forces at the spine .
simultaneously , we determind the actual trunk 
 strength required in the individual trimesters of pregnancy .
the following questions were 
 addressed : does a significant association exist between the infant s weight , the mother s 
 weight increase during pregnancy , and a specific alteration of the spine ?
twenty - six pregnant females with an average age of 30.7  4.9  years , recruited from the 
 greater rostock area of germany , participated in this study for its entire duration .
their mean body weight in the second 
 trimester was 67.8  8.0  kg , and it increase to 76.6  7.7  kg in the third trimester .
their 
 mean weight postpartum was 66.5  6.1  kg ( table 
 1table 1.characteristics of the study sample ( n = 26) sdmin  maxage ( years)30.7  4.923.043.0bmi ( kg / m ) , 2nd trimester ( n=14)24.0  3.720.032.4bmi ( kg / m ) , 3rd trimester ( n=19)26.4  3.621.134.0bmi ( kg / m ) , postpartum ( n=8)22.7  2.020.025.7parity ( n)0.5  1.005maternal weight gain ( kg ; n=17)17.1  4.01225infant s weight at birth ( g ; n=18)3,546.4  537.11,9304,490n%paritynulliparous1661.5primiparous934.6multiparous13.8family structurenuclear830.8large1246.2education 12 years830.812 years<1246.2work situationworker1661.5nonworker415.4perception of economic situationinadequate415.4adequate1661.5smokingnonsmoker1661.5quit smoking in pregnancy311.5smoker13.8previous low back pain and history of pelvic pain 
 during pregnancyyes1142.3no934.6description of low back and pelvic painburning623.1sharp415.4blunt934.6no pain13.8timing of most severe low back and pelvic painmorning27.7noon27.7end of the day1038.5night27.7all day311.5no pain13.8 ) . the study was designed as a single - center investigation of a treatment group .
pregnant 
 females were recruited from the university clinic of obstetrics and gynecology at klinikum 
 sdstadt rostock .
the 
 females were observed during their pregnancy and underwent clinical tests as detailed below . 
 immediately after the investigation period
, the females were divided into the following 
 trimesters : first trimester ( 112 weeks ) , second trimester ( 1328 weeks ) , third trimester 
 ( 2940 weeks ) , and the postpartum period ( after delivery ) .
the first trimester is mentioned 
 here for the sake of completeness , but it was not taken into account for making any 
 qualitative statements ( see the limitations of the study ) .
the inclusion criteria for the 
 clinical investigation were as follows : age above 18  years , medically confirmed pregnancy , 
 and the ability to read and write german .
the exclusion criteria were as follows : 
 pre - existing anatomical deformities of the spine , pre - existing spinal retardation , previous 
 surgery in the spine , and relevant neurological deficits .
after a detailed explanation of the purpose of the study , which was the documentation of 
 posture and determination of quality of life , each participant consented in writing to the 
 processing of her anonymized data .
the clinical investigation was approved by the ethics 
 committee of the state chamber of physicians of rostock ( registration number a 2011 
 101 ) .
the females subjective perception of pain during movement and pain at rest were scored on 
 a numeric rating scale ( nrs ) , which consists of 11 ratings from 0 ( no pain ) to 10 ( worst 
 pain imaginable)5 .
video raster stereography ( vrs ) is a computer - aided screening procedure used to generate a 
 profile of the body surface without radiation6 .
a grid of parallel lines is projected onto the dorsal surface and 
 is deformed by the surface contours of the body .
based on certain fixed points , the symmetry 
 line of the spinous processes , vertebra prominens , left and right dimples , and sacrum ) , the 
 device generates a system of coordinates corresponding to the shape of the body ( fig .
b ) three - dimensional view of the 
 dorsal surface with convex ( red ) and concave ( blue ) surface areas , vertebra prominens 
 ( vp ) , dimple left ( dl ) , and dimple right ( dr ) .
c ) spine parameters in the lateral 
 view , kyphosis angle ( ka ) , lordosis angle ( la ) , trunk inclination ( ti ) , 
 cervicothoracic ( ict ) inflection point , thoracolumbar ( itl ) inflection point , 
 lumbosacral inflection point , dimple middle ( dm ) ) . using triangulation , mathematical parameters are employed to calculate frontal and 
 sagittal planes and surface rotation automatically .
we were interested in the kyphosis angle 
 ( ka ) , the lordosis angle ( la ) , and trunk inclination ( ti ) .
video raster stereography is a 
 sensitive gauge to monitor changes in , as well as analyze , defective posture7 .
b ) three - dimensional view of the 
 dorsal surface with convex ( red ) and concave ( blue ) surface areas , vertebra prominens 
 ( vp ) , dimple left ( dl ) , and dimple right ( dr ) .
c ) spine parameters in the lateral 
 view , kyphosis angle ( ka ) , lordosis angle ( la ) , trunk inclination ( ti ) , 
 cervicothoracic ( ict ) inflection point , thoracolumbar ( itl ) inflection point , 
 lumbosacral inflection point , dimple middle ( dm ) the biomechanics of the spine have been a subject of research for several years .
some 
 models have provided valuable information about the effective forces and the stabilization 
 of forces .
one of these models was created by bergmark8 , and it is the basis of the present investigation ( fig . 2fig .
a : 
 sagittal view of the insertion of the erector spinae muscle , b : t12-l1 disk midpoint , 
 c : the combined center of gravity of upper body weight and the weight that constitutes 
 the outer load ( q ) , d : sagittal view of the origin of the global muscle , e : l5-s1 disk 
 midpoint , a : distance from the most anterior disk midpoint to the gravity line .
the 
 coordinates of the erector spinae muscle on the rib cage are expressed by c and h. c = 
 59 + 21  mm and h = 100  mm .
the coordinates of the origin d are expressed as d = 60 mm , 
  = trunk inclination ( bergmark ) ) .
the following symbols and units were used in the study : a ( mm ) , distance from the 
 most anterior disk midpoint to the gravity line ; c ( mm ) , distance from the global muscle to 
 the gravity line ; f ( n ) , power ; h ( mm ) , lever arm ; m 
 ( nm ) , moment ; x ( mm ) , x - coordinate ; and  (  ) , angle .
a : 
 sagittal view of the insertion of the erector spinae muscle , b : t12-l1 disk midpoint , 
 c : the combined center of gravity of upper body weight and the weight that constitutes 
 the outer load ( q ) , d : sagittal view of the origin of the global muscle , e : l5-s1 disk 
 midpoint , a : distance from the most anterior disk midpoint to the gravity line .
the 
 coordinates of the erector spinae muscle on the rib cage are expressed by c and h. c = 
 59 + 21  mm and h = 100  mm .
the coordinates of the origin d are expressed as d = 60 mm , 
  = trunk inclination ( bergmark ) the following indices were used in the study : g , weight force ; max lwk , maximum deflection 
 of the lumbar spine to the ventral aspect ; mp , muscle power ; n , individual component ; and n , 
 running index of the individual planes . additionally , the following parameters were 
 considered important : trunk length , vertebral body heights , vertebral body mass , 
 intervertebral disks , x - coordinate , weight force , gravitational line , muscle force , and the 
 lever arm : trunk length was determined based on the perpendicular line connecting the 
 seventh cervical vertebra ( c7 ) and the middle of the lumbar groove ( dm ) and taken as the 
 y - coordinate .
vertebral body mass was the 
 percentage distribution of the vertebral body mass from c7 to the first sacral vertebra ( s1 ) 
 is based on the data reported by duval - beaupere10 .
intervertebral disks are especially significant because 
 their actual heights are subject to the circadian rhythm .
it has been postulated that a 
 human being is 10 to 20  mm taller in the morning than he / she is in the evening .
roberts et al . reported a height 
 difference of 8 to 26  mm in young females11 .
assuming that the height of the intervertebral disks increases 
 from the cranial to the sacral aspect , we presumed a height difference of 11 to 16  mm .
to take the differences in 
 trunk length of the females into account , a compression ratio expressed in percentage was 
 introduced . in the absence of a compression ratio
compression is calculated by the following formula9 : reference height = original height  ( 1  
 compression ratio ) .
the model s trunk length can be adapted to the actual trunk length ( vrs ) 
 with the aid of the compression ratio . according to berry
, the height of the vertebral body 
 of l5 is 27  mm9 . for a pregnant woman with 
 a trunk length of 508  mm ,
weight force , that is the weight force acting on each 
 section of a segment , was the product of the mass of the section and the standard 
 acceleration of gravity ( 9.81 m / s ) .
the gravitational line is , on average , 
 located about 1  cm anterior to the midpoint of the lumbar vertebra in the most ventral 
 aspect , which , according to asmussen , is the l4 lumbar vertebra12 .
muscle force ( musculus erector spinae ) was calculated 
 under the assumption of a global center of gravity for the trunk8 .
the following equation is important for the calculation : 
 mg = fg  h. the resulting moment must be equal to the moment 
 derived from the muscle strength and the lever arm c ( fig .
, the following also apply : mg = mmp and fmp = 
 mg / c. the lever arm was the horizontal projection of the distance between the center of gravity 
 of the viewed segment and the vertebral body midpoint of the respective segment . the lever 
 arm consisted of several components .
the second was the sagittal shift of the 
 midpoint of the vertebral body in the most ventral aspect .
the third was calculated from the 
 x - shift due to trunk inclination ( angle change  ) .
this was selected in a manner such that 
 the measured point ( c7 ) concurs with the point on the model .
the fourth component was the 
 respective averaged x - value of the corresponding segment .
the lever arm was calculated by 
 the following formula : hn = a + x(max.lwk ) + x( )  
 xn .
the actual trunk strengths of pregnant females in flexion , extension , 
 lateral flexion , and trunk rotation were determined with a diers myoline device . 

the system employs the dms technique , by which isometrically introduced muscle strengths are 
 registered with a strain gauge .
simultaneously , the software incorporated in the device 
 determines the moments ( nm ) and displays them in graphic form .
data were stored and analyzed using ibm spss statistics , version 19.0 ( ibm corp .
statistics included means and standard deviations of continuous variables , frequencies , and 
 relative frequencies of categorical factors .
the level of significance was set to p < 0.05 . in case values 
 of p
< 0.05 were achieved in the friedman test , we additionally performed wilcoxon tests 
 for pairs of dependent samples .
this replaces the pairwise t - test , as the present study 
 population was a small one and the random sample was in part normally distributed and in 
 part not normally distributed .
furthermore , spearman s correlations were determined between 
 pairs of variables using the rank correlation coefficient .
back pain in pregnant females was documented separately during movement and at rest . during 
 movement , it was statistically nonsignificant throughout the entire period of pregnancy 
 ( friedman test , p = 0.717 ) .
in contrast , a change or a trend was noted in pain levels at 
 rest during pregnancy ( friedman test , p = 0.076 ) .
owing to the specific physiology of the spine during pregnancy , posture parameters can not 
 be standardized for this period of time .
comparative studies , to our knowledge , do not 
 exist . in the present investigation , the females had a kyphosis angle of 51.5  5.5 in the 
 second trimester , and the angle became even more marked in the subsequent course of 
 pregnancy . in the third trimester , the kyphosis angle was 53.5  5.8. postpartum it 
 remained at almost this level .
no significant difference in the kyphosis angle was noted at 
 the three time points of measurement ( friedman test , p = 0.105 ) .
the same was true of trunk 
 inclination ( friedman test , p = 0.779 ) .
in contrast , changes in the lordosis angle 
 throughout the observation period showed a statistical trend ( friedman test , p = 0.057 ) . 

spearman s test revealed a positive correlation between trunk inclination and weight in the 
 second trimester (  = 0.534 , p = 0.049 ) , and a negative correlation between trunk 
 inclination and the kyphosis angle in the third trimester (  = 0.520 , p = 0.023 ) .
weight and the kyphosis angle were 
 also significantly correlated in the third trimester (  = 0.479 , p = 0.038 ) .
the calculations obtained from the biomechanical model clearly show that the global moment 
 increased a little from the second to the third trimester ( 10.4 nm  1.8 nm vs. 11.0  3.9 
 nm ) , but dropped markedly postpartum ( table 
 2 table 2.force values sdmin  maxflexion ( n)2nd trimester ( n=8)69.2  33.824.0131.03rd trimester ( n=3)66.0  30.347.0101.0postpartum ( n=8)84.9  45.026.0150.0extension ( n ) 2nd trimester ( n=8)93.0  62.342.0242.03rd trimester ( n=3)77.7  34.538.0101.0postpartum ( n=7)76.4  42.033.0159.0trunk rotation on the left side ( n)2nd trimester ( n=8)57.6  21.531.588.03rd trimester ( n=6)27.2  6.819.036.0postpartum ( n=7)51.3  15.041.084.0trunk rotation on the right side ( n)2nd trimester ( n=8)66.8  34.938.0130.03rd trimester ( n=6)37.2  23.217.081.0postpartum ( n=7)47.3  15.432.079.0lateral flexion on the left side ( n)2nd trimester ( n=7)122.5  40.276.0190.03rd trimester ( n=7)97.3  72.017.0226.0postpartum ( n=7)102.3  43.352.0167.0 lateral flexion on the right side ( n)2nd trimester ( n=8)125.1  54.159.0199.03rd trimester ( n=7)84.9  47.947.0182.0postpartum ( n=7)100.6  37.576.0162.0torque segmental * ( nm)2nd trimester ( n=10)15.1  2.39.617.73rd trimester ( n=17)16.1  2.112.219.4postpartum ( n=7)13.9  1.412.716.2torque global * ( nm)2nd trimester ( n=10)10.4  1.87.713.03rd trimester ( n=17)11.0  3.95.519.2postpartum ( n=7)8.0  2.35.210.5muscle strength , erector spinae muscle * ( n)2nd trimester ( n=10)130.3  22.696.5163.03rd trimester ( n=17)137.2  48.769.3240.2postpartum ( n=7)99.5  28.765.4131.0diers myoline device ; * calculations based on the 
 biomechanical model ) . therefore , the strength of the musculus erector spinae , which marks the 
 limit of the global moment , increased from the second to the third trimester ( 130.3  22.6 n 
 vs. 137.2  48.7
diers myoline device ; * calculations based on the 
 biomechanical model comparison of the theoretically determined global and segmental moment showed that the 
 segmental moment was a little higher than the global moment . during pregnancy , the segmental 
 moment increased initially from the second to the third trimester and then decreased 
 postpartum ( table 2 ) .
the measurements obtained with the diers myoline device showed that there was 
 an increase in strength in flexion , especially after parturition ( third trimester 66.0 n  

the strength level determined in the second trimester 
 started to fall when the females approached the third trimester ( table 2 ) . in extension ,
the pregnant females achieved the greatest strength in the second trimester , 
 which they were unable to achieve in the third trimester and postpartum ( second trimester 
 93.0 n  62.3 n , third trimester 77.7 n  34.5 n , pp 76.4 n  42.0
the strength values in rotation ( left / right ) revealed that the increase in strength started 
 to fall as pregnancy progressed towards the second trimester and fell markedly as pregnancy 
 progressed towards the third trimester .
overall , the strength level achieved in the second trimester was not 
 achieved postpartum in any of the measured directions ; the sole exception was flexion .
according to martins et al . , back pain occurs in approximately 80% of pregnant females and 
 is mainly located in the lumbar and sacroiliac region13 .
the 
 following factors are known to influence it : number of births , preexisting back pain prior 
 to pregnancy , body mass index , weight increase , and muscular dysfunction14,15,16 .
parity was no major concern in the 
 present investigation as , 61.5% of the females were nulliparous while a mere 3.8% were 
 multiparous .
the estimates concerning back pain during movement and at rest were 
 interesting , as they permit potential conclusions about muscular dysfunction . in the present 
 clinical study ,
back pain at rest increased markedly from the second to the third trimester , 
 whereas pain during movement remained unchanged .
one explanation for this phenomenon could 
 be that , due to hormonal changes , the proprioceptors of tendons , muscles , and ligaments do 
 not respond adequately .
solomonow et al . reported that stimulation of mechanoreceptors in 
 the supraspinal ligament , the zygapophyseal joints , and the intervertebral disks evokes a 
 reflex response in the musculus multifidus17 .
according to richardson and hides , contraction of spinal muscles 
 close to joints stiffens their passive structures and thus indirectly improves the ability 
 to perceive movement18 .
such mechanisms 
 are frequently discussed in chronic back pain and should be taken into account in pregnant 
 females as well19,20,21 . another factor discussed in connection with back
the females in the present investigation were on average 
 30.7  4.9  years old , and thus a little older than the average age of 29.2  years for 
 primiparous females as reported by the federal statistical office of germany .
in addition to 
 age , sjodahl et al . mentioned that the endurance of the trunk muscles influences pain23 .
what measures help to relieve back pain during pregnancy ? in an investigation comprising 
 105 pregnant females aged on average 30.7  4.0  years , haakstad et al
. concluded that a 
 60-minute fitness program performed twice a week achieves a nonsignificant reduction of back 
 pain and pelvic girdle pain24 . in a 
 review article , mazzarino et al .
mentioned that the impact of pilates exercises on the 
 health of pregnant females has not been fully clarified25 . a review article published by close et al .
indicates that 
 alternative therapies have a rather minor effect on pelvic and back pain in pregnant 
 females26 .
the authors report that physical activity during pregnancy 
 influences the quality of life in women and that acupuncture reduces back pain more markedly 
 than conventional precautionary measures . in a cross - sectional investigation comprising 1279 
 pregnant females , nascimento et al . concluded that an active lifestyle is extremely 
 important29 .
however , the ideal type of 
 physical activity is not clear . walking and water gymnastics are the most common forms of 
 activity , since they have practically no contraindications29 . in water gymnastics ,
the hydrostatic pressure of water promotes 
 venous return of blood , which in turn exerts a positive impact on cardiovascular parameters . 

the simultaneously enhanced renal perfusion causes greater diuresis and natriuresis , which 
 reduces pregnancy - related edema2 . with 
 regard to the activation of deep segmental muscles and the concomitant reduction of pain
, 
 the loss of gravity appears to be rather disadvantageous because these muscles require 
 constant input from the proprioceptors .
therefore , we assume that such activation is best 
 achieved by performing weight - bearing exercises with one s own body weight on an unstable 
 surface ( pezzi ball , sling training ) .
the existing body of data on this subject is still 
 meager ; further research is needed on these issues .
the use of raster stereography in pregnant females has been poorly investigated so far . 

reference values determined by schrder et al . for healthy males and females aged 19 to 
 40  years have been used to study the variance of posture parameters30 .
the lordosis angle in males ( 36  7 ) was less pronounced than that in females ( 43 
  8 ) . in the present study ,
the kyphosis 
 angle in pregnant females may differ by more than 3.5 from that of nonpregnant females .
it was most marked in the second trimester ( 51.1  
 9.1 ) and started to reduce postpartum ( 48.4  7.9 ) ( table 3table 3.results2nd trimester(n = 14)3rd trimester(n = 19)postpartum(n = 8)painat rest1.4  2.02.6  2.41.6  1.8during movement2.4  2.42.7  2.32.9  2.9posturekyphosis angle51.5  5.553.5  5.854.1  4.6lordosis angle51.1  9.149.4  6.848.4  7.9trunk inclination8.8  16.211.8  18.115.1  19.9data shown as means  sd .
data shown as means  sd . on the nrs from 0 to 10 . in 
 degrees . * in  mm .
performed raster stereographic measurements in 19 pregnant and 15 nonpregnant 
 females31 . in the second and third 
 trimester , they observed increases of 0.83 and 1.02 in the angles of curvature of the 
 thoracic and lumbar spine , respectively . compared with the control group ,
however , the authors assume that back pain , 
 which is mainly observed in the first and second trimester , can not be explained by a change 
 in the curvature of the spine .
our calculations obtained from biomechanical models indicate 
 the same result . assuming that the body has a global center of gravity , the moment remains 
 at a similar level from the second to the third trimester ( table 2 ) .
therefore , the force exerted by the global system of the 
 musculus erector spinae to stabilize the entire system hardly differs in the second and 
 third trimesters .
viewing the segments separately , the distance between the center of 
 gravity and the center of the vertebral body differs at every level . viewed globally ,
the 
 mean of the individual segments is not equivalent to the lever arm . therefore , the total 
 moments are somewhat higher .
it is possible that in the individual segments , the deep muscle 
 layers have to expend more energy to maintain a balance between the active moment and the 
 local muscles . as regards the spinal curvature , okanishi et al .
reported that pregnant 
 females tend to develop lumbar kyphosis , which is associated with specific symptoms32 .
we observed the same result , especially 
 in the period of transition from the second to the third trimester . to our knowledge , the active moment on the spine of pregnant females has not been 
 investigated so far in a biomechanical model .
trunk strength in all directions of motion was higher in the second trimester than it was 
 in the third trimester .
it is possible that greater 
 production of growth hormones in combination with weight increase , both of which occur in 
 pregnant females , cause the adaptation of muscles33 .
however , the muscles are able to unfold their strength effectively 
 only when they are directed optimally , and this does not occur until the end of pregnancy . 

the main limitation of the study is the small sample size , which hindered the use of 
 complex statistical procedures as well as the pronouncement of generalized statements .
most of all , a sufficient number of pregnant females in the first trimester could not be 
 recruited for the study .
a large number of flyers was sent to the offices of gynecologists 
 and midwives in the greater rostock area .
the reasons for the limited number of females in 
 the first trimester are not clear .
it could have been due to poor interest in scientific 
 investigations , anxiety , or  inadequate  financial remuneration , or because the doctors and 
 midwives lacked information about the clinical trial .
thus , the recruitment was largely 
 confined to pregnant females in the second and third trimester .
the biomechanical model was 
 also a limitation of the study , specifically the actual distribution , stratification , and 
 insertion of muscles in the individual segments .
furthermore , the height of vertebral bodies 
 and intervertebral disks and distance  a  were taken from the published literature .
our conclusions are as 
 follows : greater attention must be given to improving physical capacity in pregnant 
 females27 , 34 , 35 .
back pain at rest can 
 be alleviated by moderate strength and stabilization exercises1 , 36,37,38 .
the marked weight 
 increase in the third trimester is closely related to the kyphosis angle in pregnant 
 females .
 [ purpose ] the purpose of this single - center investigation was to study the impact of 
 pregnancy on back pain and body posture . 
 [ subjects ] the subjects were 26 pregnant females . 
 [ methods ] data were generated with a spine scanner ( diers formetric 4d ) , trunk 
 strength measurement ( diers myoline ) , a numeric pain scale ( 0 to 10 ) , and a 
 biomechanical model . 
 parameters were compared during each trimester . [ results ] 
 the 
 alteration in pain level at rest and lumbar lordosis angle in the females revealed a 
 statistical trend during pregnancy . 
 spearman s test showed positive correlations between 
 body weight and trunk inclination during the second trimester , and between body weight and 
 the kyphosis angle in the third trimester . the trunk inclination and 
 the kyphosis angle 
 revealed a negative correlation in the third trimester . 
 based on our analysis , the highest 
 moments and muscle strength must be expended in the third trimester . 
 the actual muscle 
 strength is greatest in the second trimester . 
 [ conclusion ] pain at rest must be given 
 greater attention in pregnant females , and their increasing lumbar kyphosis must be 
 counteracted . 
 exercising the deep segmental muscles may serve as a preventive measure . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION