Subjects

The sample included 22 twin pairs with dichorionic placentas and four pairs with monochorionic placentas. This study was approved by the local Ethics Committee and informed consent was obtained from each patient. Women were excluded from the study whether there were any maternal or fetal complications, including hypertension, diabetes mellitus or a fetal structural anomaly on ultrasound. Co-twins with discordant or restricted growth during gestation were also excluded. The characteristics of pregnancies are presented in Table 1. Perinatal outcomes are summarized in Table 2.

Table 1 Characteristics of pregnancies Full size table

Table 2 Perinatal outcomes of twins Full size table

In twenty-two patients, the twins were dizygotic (dichorionic); of them 13 twin pairs were of different sexes, five were both females and four were both males. Of the four monozygotic twin pregnancies, two were dichorionic and two were monochorionic, three were both females.

Instruments

All 4D examinations were carried out using the Voluson 730 imaging system (GE/Kretz-Technik, Zipf, Austria). All ultrasound examinations were initiated as 2D scans using an electronic transducer with 3D/4D facility: a 4 to 8 MHz transducer for trans-abdominal imaging and/or 5 to 9 MHz transducer for trans-vaginal sonography.

Procedure

Evaluation of fetal motor activity was carried out during ultrasound nuchal translucency scan at late first trimester and early second trimester (11 to 14 weeks’ gestation). The more active fetus in each pair was noted according to the position and/or sex without reporting to parents. Measures of fetal motor activity (frequency, duration and number of movements) were collected from DVD recordings of two separate periods of 10 min during twins’ scan. An animation software (Jasc Animation Shop 3) was used for sampling the sequence of frames during the observation period (Figure 1). One investigator (SD) analyzed the measures of activity. Assessment was repeated thrice for each recording to test reproducibility. Intra-observer variations for repeated measurements, were 4.3%±0.4. No significant difference was found in the mean of repeated measurements and the characteristics of the regression line indicate reliability: the slope was close to 1 and the intercept did not differ from 0.

Figure 1 4D imaging sequence (frame by frame analysis) of fetal movements at 12-weeks gestation (a–j). (rate: 16 frames per second). Full size image

The measures of activity were assessed according to the following definitions: frequency, calculated for isolated arm and leg movements, stretching, head and general body movements; duration, the fraction of time of any movement during scanning period; number of movements, total count of any movement during each observation period.

No significant difference was found in comparing the two observation periods for fetal movements (P=0.29 for total movements count).

Reported maternal perception of the more active twin was documented later, at the mid-trimester anatomical scan.

Using a modification of Rothbarts’ Infant Behavior Questionnaire (IBQ)6 and revised IBQ,7 measures of postnatal twins’ temperament were obtained from questionnaires completed by the twins’ mothers at 3 and 6 months of age. The IBQ has been designed to measure temperament in infants between the ages of 3 and 12 months by assessment of the following dimensions of temperament: activity level, distress to limitations, approach, fear, duration of orientation, smiling and laughter, vocal reactivity, perceptual sensitivity, high intensity pleasure, low intensity pleasure, cuddliness, soothability and falling reactivity/rate of recovery from distress. Score of 0 to 3 was given for every measure of temperament.

The number of fetal movements was expressed as the number of movements during the observation period of 20 min.

Statistical analysis

All observable movement patterns recorded were presented collectively with maximum, minimum and median frequencies during this period. Owing to the fact that the data did not follow a normal distribution, the non-parametric Wilcoxon rank-sum test was used for comparison of the frequencies of fetal movements observed in each pair. The spearman rank order correlation was used for the correlation of movement frequency in fetuses and infant's temperament scores.

The computer program MedCalc for Windows (Version 9.6, 1993 to 2008, Belgium), was used for analysis.

The results were considered statistically significant when P-values were <0.05.

Prediction of the more active twin using ultrasound as compared with maternal perception at mid-trimester was evaluated by sensitivity, specificity, predictive values, accuracy and likelihood ratios. The positive predictive value (post-test probability of a positive test); negative predictive value (post-test probability of a negative test); likelihood ratio of a positive test (how much more likely is a positive test to be found in a person with the tested condition than in a person without it); and accuracy, what proportion of all tests has given the correct results (true positive and true negative as a proportion of all results). The receiver operating characteristic curve (or ROC curve) was used to evaluate the above-mentioned tests. The ROC curve is a plot of the true positive rate against the false positive rate for the different possible cutpoints of a diagnostic test. The curve shows the tradeoff between sensitivity and specificity (any increase in sensitivity will be accompanied by a decrease in specificity). The closer the curve follows the left-hand border and then the top border of the ROC space, the more accurate is the test. The closer the curve comes to the 45 ° diagonal of the ROC space, the less accurate the test. The slope of the tangent line at a cutpoint gives the likelihood ratio for that value of the test. The area under the curve is a measure of text accuracy.

The end result of a more active twin was defined by the score according to the postnatal questionnaire.