title: Assessment of the accuracy of estimation of gestational age in cattle from placentome size using inverse regression.
abstract: OBJECTIVE
  To assess, in dairy cows between 60-160 days of gestation, whether the agreement between actual gestational age and that predicted by placentome size could be improved by using individual placentome measurements rather than a single average, and to identify how increasing the number of placentomes measured improved the prediction.
  
  
  METHODS
  Data were obtained from 58 cows examined using transrectal ultrasonography every 10 days between 60-130 days, and every 15 days between 130-160 days of gestation. For each cow, four to six placentomes in each of the pregnant and non-pregnant horns were examined from the region of the uterus near to the cervix. A mixed-effects model, which included cow as a random effect, and a simple linear regression which ignored within-cow correlation, were fitted to the data. Inverse regression was used to compare the 95% prediction bands obtained for estimating gestational age using the means of three, five or eight placentome measurements.
  
  
  RESULTS
  The fit of the mixed effects model was better than a simple linear regression (p<0.001) but the fitted lines from the two models were very similar. Using the simple regression model, for a mean placentome length of 15 mm estimated gestational age was 100 days, with 95% CI of 68-131 days for measurement of three placentomes, and 80-120 days for eight placentomes.
  
  
  CONCLUSIONS
  The agreement between placentome size and gestational age was improved by increasing the number of placentomes measured. Direct comparison of these CI with those for other published fetal measures was not possible as similar prediction bands have not previously been calculated for fetal measures; however one study reported a residual SD which had been calculated using size measurements as the predictor variable and age as the response variable. Using these data 95% CI were calculated to be ±9 days for crown-rump length and ±25 days for uterine diameter. These are likely to be an underestimate of the true CI and do not take account of the increase in variance of the difference between predicted and actual gestational age as gestational age increases.
  
  
  CONCLUSIONS
  Placentome measurement may be useful for estimating fetal age in late gestation. Further research is required to better establish the agreement between gestational age and the fetal measurements which are currently used to estimate fetal age.
authors.affiliation: a Institute of Veterinary, Animal and Biomedical Sciences , Massey University , Private Bag 11 222, Palmerston North 4442 , New Zealand.
authors.forename: K E
authors.initials: KE
authors.lastname: Lawrence
authors.affiliation: a Institute of Veterinary, Animal and Biomedical Sciences , Massey University , Private Bag 11 222, Palmerston North 4442 , New Zealand.
authors.forename: F D
authors.initials: FD
authors.lastname: Adeyinka
authors.affiliation: a Institute of Veterinary, Animal and Biomedical Sciences , Massey University , Private Bag 11 222, Palmerston North 4442 , New Zealand.
authors.forename: R A
authors.initials: RA
authors.lastname: Laven
authors.affiliation: b Institute of Fundamental Sciences , Massey University , Tennent Drive, Palmerston North 4474 , New Zealand.
authors.forename: G
authors.initials: G
authors.lastname: Jones
mesh_terms: D000818:Animals; D002417:Cattle / Q000502:physiology*; D005260:Female; D005865:Gestational Age; D010920:Placenta / Q000502:physiology*; D011247:Pregnancy; D011270:Pregnancy, Animal; D014463:Ultrasonography / Q000662:veterinary*
keywords: Transrectal ultrasonography; gestational age; inverse regression; placentome length; prediction interval