Epidemiological studies identify birth after 41 weeks or after 42 weeks as a risk factor for a variety of adverse neonatal outcomes. Caughey et al.  conducted a retrospective, cohort study of all low-risk, term, cephalic, and singleton births that were delivered at the University of California, San Francisco, between 1976 and 2001. The incidence of adverse neonatal mortality outcomes at 40, 41 and 42 weeks' gestation was examined in a multivariate analysis and compared with the rates in pregnancies that were delivered at 39 weeks of gestation, after controlling for maternal demographics, length of labour, induction, mode of delivery, and birth weight (except macrosomia). Compared with the outcome at 39 weeks' gestation, the relative risk of meconium aspiration increased significantly from 2.18 at 40 weeks to 3.35 at 41 weeks and 4.09 (95% CI 2.07-8.08) at 42 weeks. A composite outcome of 'severe neonatal complications' including skull fracture and brachial plexus injuries, neonatal seizures, intracra-nial haemorrhage, neonatal sepsis, meconium aspiration syndrome, and respiratory distress syndrome, increased from a relative risk of 1.47 at 40 weeks to 2.04 at 41 weeks to 2.37 (95% CI 1.63-3.49) at 42 weeks.
A strong association between neonatal seizures and delivery at 41 weeks' gestation or more has been identified in previous case control studies. Minchom et al.  found that delivery after 41 weeks' gestation was associated with an odds ratio of 2.7 (95% CI 1.6-4.8). Curtis et al.  studied 89 babies with early neonatal seizures delivered after 42 weeks' gestation in Dublin. Twenty-seven were delivered after 42 weeks' gestation compared with 6 of 89 controls (odds ratio 4.73; 95% CI 2.22-10.05).
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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.