Preterm labour is not a single disease entity but is a symptom or a syndrome which may have one or more of a number of causes (Fig. 21.3). Preterm labour has been linked to cervical incompetence, abnormalities of haemostasis, infection within the uterus, placental abruption or decidual haemorrhage, fetal or maternal stress and multiple pregnancy. In some cases several of these factors may act together to increase the likelihood of preterm delivery or to affect the gestational age at which preterm delivery occurs. So, for example, twin pregnancies deliver at 36 weeks. Multiple pregnancy probably leads to preterm delivery through at least two mechanisms. Over-distension of the uterus leads to premature up regulation of contraction associated proteins and of factors which mediate cervical ripening, all of which have been
Delivery within 24 hours Delivery within 48 hours Delivery within 7 days
Fig. 21.3 Meta-analysis of the effect of tocolytic administration upon preterm delivery and neonatal outcomes. Adapted from Gyetvai K et al. Obstet Gynecol. 1999 94(5 Pt 2): 869-77.
shown to be sensitive to mechanical stretch. Multiple pregnancy is associated with multiple placentas and therefore with an earlier rise in placental CRH concentrations in the circulation. A preterm delivery in twins at 28 weeks will not be due simply to the multiple pregnancy and must have another aetiology associated with it, for example, infection or cervical weakness. Had the same pregnancy been a singleton pregnancy it is probable that the preterm delivery would have occurred at a later gestational age.
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The first trimester is very important for the mother and the baby. For most women it is common to find out about their pregnancy after they have missed their menstrual cycle. Since, not all women note their menstrual cycle and dates of intercourse, it may cause slight confusion about the exact date of conception. That is why most women find out that they are pregnant only after one month of pregnancy.