Since 1 April 1968, the civil registration system in Denmark has assigned an individual, unique registration number to all citizens. This number permits accurate linkage of information from different registries. A research database of parity was established with data from this registration system, including information on all live births to women born between 1 April 1935 and 31 March 1978. This is described in detail elsewhere.13
For our study, we added mandatory reported information on fetal death (spontaneous abortion, hydatidiform mole, ectopic pregnancy, and stillbirth) and induced abortion from three national health registries. The national discharge registry, established in 1977, comprises diagnoses at discharge for all patients admitted to hospital. From this registry we obtained all cases of spontaneous abortion, hydatidiform mole, and ectopic pregnancy. The medical birth registry, established in 1973, contains information on all births in Denmark. From this registry we obtained information on stillbirths. Information on induced abortions was obtained from the national register of induced abortions. We identified all reproductive outcomes in Denmark in the period 1978-92. Reproductive outcome was defined as live birth, stillbirth, spontaneous abortion including hydatidiform mole, ectopic pregnancy, or induced abortion.
We restricted analyses concerning the combined effects of maternal age and reproductive history on outcome of pregnancy to pregnancy outcomes in the period 1988-92. In these analyses reproductive history was exact parity status (complete information on births) and complete information on other reproductive outcomes in the preceding 10 years.
Maternal age at conception was estimated by deducting gestational age at birth from maternal age at delivery. Gestational age was recorded for more than 90% of the live births and stillbirths, and for the remaining cases we applied the mean gestational age for the entire population. As gestational age was not recorded for spontaneous abortion, hydatidiform mole, and ectopic pregnancy, we set this at 9 weeks, 12 weeks, and 8 weeks respectively.
According to Danish standards, a stillbirth is defined as the birth of a child with a gestational age of 28 weeks or more who does not show any sign of life. Parity was defined in two groups: nulliparous women (no previous live births or stillbirths) and parous women.
We estimated the risk of fetal loss according to maternal age as a proportion of all pregnancies intended to be carried to term—that is, live births, stillbirths, spontaneous abortions, and ectopic pregnancies. For the risk of stillbirth according to maternal age, only pregnancies at risk of becoming a stillbirth were taken into consideration, and consequently the risk constitutes the proportion of stillbirths among all births.
The number of pregnancies intended to be carried to term might be slightly biased because some fetal losses occurred before an intended induced abortion. Such cases would wrongly be counted as intended pregnancies. To evaluate this bias we estimated an adjusted number of fetal losses by deducting the expected number of pregnancies that ended as a fetal loss before an intended induced abortion from the total number of fetal losses. The adjusted risk of fetal loss was calculated as the adjusted number of fetal losses divided by the number of live births plus adjusted number of fetal losses.