As mentioned above, the denominator used is paramount when considering complications of term and postterm pregnancies. The effects of utilizing the wrong denominator in the setting of antepartum stillbirth are primarily: 1) the absolute rate of antepartum stillbirth is increased when the smaller, inappropriate, denominator is utilized (Figure ); and 2) the relationship of the relative rates of stillbirth by week of gestation is distorted. This problem has been described by several authors [4
] and was excellently summarized by Dr. Gordon Smith in 2001 [7
Figure 1 Comparing the denominators of pregnancies delivered (PD) and ongoing pregnancies (OP). Pregnancies delivered consists of just those women delivered during a particular week of gestation. Ongoing pregnancies consists of both those women delivered during (more ...)
In one of the earlier commentaries on this problem, Yudkin et al [3
] noted in 1987 that if the risk of antepartum stillbirth was considered by week of gestation using the denominator of total live births occurring at that gestational age, the risk seemed highest among preterm patients as they had the smallest denominators. However, when they utilized the denominator of ongoing pregnancies, they found that the rate of antepartum stillbirth decreased until 39 weeks of gestation, then increased through 41–42 weeks of gestation. These findings have been reproduced by other authors and using data from a paper published in 2003, the effect of the denominator on antepartum stillbirth is demonstrated (Table ) [8
]. To be more precise, Dr. Smith introduced the idea that since the women who deliver during a particular week are not actually at risk for antepartum stillbirth the entire week, a correction factor can be applied in the formula:
Antepartum Stillbirth by Week of Gestation Using Different Denominators
PAn = An/[Pn - (0.5 * Bn)]
PAn is the risk of antepartum stillbirth in given week of gestation n
An is the number of antepartum stillbirths occurring during the week of gestation n
Pn is the number of pregnancies which start week n (ongoing pregnancies)
Bn is the number of pregnancies delivered during week n
The importance of this correction factor can be seen in the results of Table , which are calculated based on the numbers from the 2001 paper [6
]. Similar to Table , there is an obvious difference in the overall trend when examining the antepartum stillbirth rates by PD and OP. However, when using the correction factor, the rise in antepartum stillbirths occurs earlier and appears to be more dramatic.
Antepartum Stillbirth by Week of Gestation Using the Correction Factor for Ongoing Pregnancies
Even the formula above, which is an improvement upon others' work in this field to define the appropriate measure of risk of antepartum stillbirth, has two flaws that can be rectified with better data. The first is that while the use of the correction term will subtract off one half of the births in that particular week in order to get a better estimate of ongoing risk, in any particular week of pregnancy, the number of births per day is unlikely to be a uniform distribution. Rather, like the overall distribution of gestational ages, the number of births will increase prior to 40 weeks of gestation and decrease thereafter. Thus, the exposure to the women delivered at a given week of gestation is likely to be higher prior to 40 weeks of gestation and lower after this threshold. Furthermore, the correction term will lead to a falsely larger denominator for the women prior to 40 weeks of gestation and a smaller denominator for women after 40 weeks of gestation.
The second problem is that while we utilize the day that the patient delivers as the gestational age of the antepartum stillbirth, by definition, the stillbirth may occur prior to the day of delivery. Just how much prior to delivery stillbirth occurs may lead to a lag effect in the difference in the estimated rates. For example, if all of the antepartum stillbirths occurred, on average, a week prior to their delivery, then the rates would necessarily increase a week earlier than currently estimated. Thus, a better approximation of the gestational age of stillbirth would be to generate an estimate of the date at which stillbirth occurs using the date(s) of the last known viable heart rate, last known fetal movement, and the date the stillbirth was documented. Of note, this estimate improves at term when women are often seen at least once a week and are usually asked to report the absence of fetal movement.
Even among those who are interested in improving the antepartum risk assessment of stillbirth, there is disagreement on how to measure and utilize this risk. In a controversial paper on the topic, Cotzias et al used ongoing pregnancy as the denominator, but changed the numerator to include all current and future stillbirths to generate a prospective stillbirth risk [9
]. Using this equation, they found that the prospective stillbirth risk was higher with preterm gestations, decreased in term pregnancies until 40 weeks of gestation, then increased through 42 weeks of gestation. They used these calculations to suggest that induction of labor should be considered with the onset of fetal pulmonary maturity at 38 weeks of gestation. The responses to this article found the use of this calculation objectionable as the rate of stillbirth derived from such estimation does not consider the length of time of exposure, nor does the suggestion of early induction of labor consider the costs or the marginal, incremental benefits that might be incurred or achieved [10
While it is clear that ongoing pregnancies should be utilized in the denominator for antepartum stillbirth as these are the women at risk, it is less clear whether other maternal or neonatal complications of pregnancy should be considered in the same way. Consider neonatal and perinatal deaths (stillbirths plus neonatal deaths). In order for a neonatal death to occur, the fetus had to become a neonate by being born. Thus, the fetuses of ongoing pregnancies are not yet at risk. This leads to the rate that has been used historically, that is, the number of neonatal deaths divided by the number of live births [6
]. Therefore, this is a problem with the metric proposed by Joseph [3
]. In a recent paper which suggests combining stillbirths plus neonatal deaths into one outcome, it is demonstrated that such a metric finds the risk of perinatal death to rise beyond 35 weeks of gestation, supporting possibly earlier induction of labor. Unfortunately, while at term, such decision-making based on the ongoing risk of stillbirth is sensible, causing more preterm births which are themselves associated with higher rates of morbidity and mortality may not be supported by such algebraic manipulation. A better measure of the total ongoing risk to a pregnancy was proposed by Smith as what he describes as the cumulative probability of perinatal death which combined both stillbirth and neonatal deaths [7
What about measures of neonatal morbidity? These might include five-minute Apgar scores less than 4 or 7, admission to the neonatal intensive care unit, birth trauma, or intrauterine growth restriction (birthweight < 3rd, 5th, or 10th percentile). For all of these outcomes, the neonate cannot experience the outcome until birth, so the standard measure utilizing pregnancies delivered should apply. The birthweight metric should also be examined using pregnancies delivered, since one does not know the actual birthweight until the infant is delivered. Interestingly, one could use the estimated fetal weight from sonographically predicted birthweight to generate risks of intrauterine growth restriction. If one did perform weekly ultrasounds of an entire population of pregnant women, then, and only then, could ongoing pregnancy be used as the denominator.
Again, Joseph uses "revealed SGA" as the number of SGA fetuses born at a particular gestational age over a denominator of all ongoing pregnancies [3
]. This metric, will of course increase over time as the denominator shrinks with further deliveries, so again provides little insight to an optimal time of delivery. The only types of neonatal morbidities that can be measured using ongoing pregnancies as a denominator would be those that can be measured in all pregnancies. For example, if we routinely measured all in utero fetuses by ultrasound and identified the SGA fetuses, then the proper denominator would be ongoing pregnancies. However, since we wouldn't deliver these SGA fetuses before 34–36 weeks of GA, the numerator would have to be the SGA fetuses identified by ultrasound, not delivered.
For maternal complications of pregnancy, it is more difficult to identify the group of women at risk for various complications of pregnancy. There are some complications which clearly occur in women who are antepartum (Table ). For example, preeclampsia is usually an antepartum diagnosis. Thus, it makes sense to utilize ongoing pregnancies as the denominator when estimating the risk of preeclampsia at a particular week of gestation [2
]. For mode of delivery (cesarean, vaginal, or operative vaginal) and outcomes related to the mode of delivery such as wound complications or perineal lacerations, clearly pregnancies delivered should be used as the denominator. However, there are a number of complications that can occur either prior to the onset of labor or during labor such that determination of the appropriate denominator to be used may be difficult. For example, chorioamnionitis occasionally occurs prior to the onset of labor. In this setting, its risk should be determined based on all women at risk, i.e., ongoing pregnancies. However, the majority of chorioamnionitis occurs during labor, and thus would only apply to the pregnancies delivered during a particular week of gestation. The same holds true for placental abruption, which can occur prior to the onset of labor, but its risk is increased during labor. When attempting to describe these risks, it is important to specify what aspect of the complication is being examined.
Appropriate Denominator* when Considering Pregnancy Outcomes and Complications by Week of Gestation
Of note, in studies that examine perinatal morbidity by gestational age at term, a number of complications appear to increase with increasing week of gestation beyond 38 to 39 weeks. Complications which do increase by week of gestation at term include neonatal outcomes such as acidemia and macrosomia as well as maternal morbidities such as preeclampsia, postpartum hemorrhage, perinatal infection, and cesarean delivery [2