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Sex differences in fetal mouth movements

1997; Elsevier BV; Volume: 350; Issue: 9094 Linguagem: Inglês

10.1016/s0140-6736(05)63635-5

1474-547X

Peter G. Hepper, E Alyson Shannon, James Dornan,

Oral and Craniofacial Lesions

Sex differences in behaviour have been widely studied after birth, whereas behavioural studies of the fetus have largely overlooked sex differences, although it has been reported that female fetuses habituate earlier than males. However, given the presence of sex differences in the newborn infant, the current view of continuity of development would suggest that sex differences do not appear at birth but have their developmental origins in the prenatal period. We have observed one sex difference present before birth. Fetuses of 39 pregnant women were examined. All were singleton pregnancies with no obstetric problems and all resulted in normal healthy babies at term. 20 women gave birth to female offspring, 19 to male. Ethical approval for the study was given by the Research Ethics Committee, Faculty of Medicine, Queen’s University of Belfast. All women gave written informed consent to take part in the study. Women were scanned at 16, 18, and 20 weeks of gestation as part of a study examining the behaviour of the fetus at the same age as its routine ultrasound scan to check for structural anomalies (16–20 weeks). These observations were done separately from their anomaly scan. Fetuses were observed for 60 min with an Ultramark 4 plus or Dornier AI3200 ultrasound scanner. A longitudinal view of the fetus’s body and head was obtained and this was recorded on videotape for later analysis. The videotapes were replayed and the number of mouth movements exhibited by each fetus at each gestational age counted. Mouth movements were defined as movements of the jaw which may or may not include tongue movements. A mixed design analysis of variance was done for the betweensubject factor of sex (male or female) and within-subject factor of gestational age (16, 18, 20 weeks of gestation). Mouth movements increased with advancing gestational age (p<0·001, see figure). Females were found to move their mouths more than males (p=0·021, figure). Mouth movements per se may have little function for the fetus but are essential for the newborn infant to enable it to suck, gain nourishment, and hence survive. It is therefore not surprising that such movements appear early in gestation and occur frequently thus ensuring their adequate development by the time of birth. Our results indicate a sex difference in behaviour of the human fetus from early in gestation. Females are developmentally more advanced than males, both preterm and at term, and the difference observed here may reflect a more general ontogenetic pattern of advanced female development compared with males, perhaps beginning at conception. The observation of a sex difference in behaviour at such an early age favours a biological explanation of its cause although the proximate mechanism remain(s) to be determined. The presence of sex differences in the behavioural development of the fetus may reflect a more fundamental difference in the developmental pattern of males and females. Futher investigations are underway to explore possible differences. However, researchers should be aware of the potential for differences in behaviour according to fetal sex.