Figure 6.

a Fitted seasonal hazard ratios as a function of time of year of conception and time of year of ongoing pregnancy, with adjustment for smoking, parity, relationship status, age, education and interactions between education and smoking, parity and relationship status. The shading indicates the magnitude of the fitted hazard ratio for birth, as shown in the shading key. The axes show dates of conception as horizontal and dates of ongoing pregnancy as vertical. These results are based on restricting analysis to time since 1998 when more accurate assessment of gestational age was routinely based on ultrasound and smoking status was known (n = 443 566). The bands are narrow because for each day of the year of conception the pregnancy is at risk of preterm for only 15 weeks. b. As for a, except that medical interventions to end the pregnancy are treated as censoring events.