Assumptions used in estimating a population benchmark
The assumptions used in estimating a benchmark for referral for psychological therapies of 80 per 1000 deliveries and referral to a specialist perinatal mental health service of 40 per 1000 deliveries are based on the following sources of information:
- epidemiological data on common mental health disorders in the general population
- current detection rate of mental disorders in the antenatal and postnatal period
- expert clinical opinion of the topic-specific advisory group, based on experience in clinical practice and literature review.
According to the report NHS maternity statistics, England: 2005-06, about 593,400 NHS hospital deliveries took place in England in 2005-06. A further 15,900 deliveries took place at home. This suggests that there were around 609,300 deliveries in England in 2005-06.
For the purpose of this benchmark, the base population is the number of deliveries in England in 2005-06. Delivery data are also used in the associated commissioning tool. It has been chosen for the following reasons:
- prevalence estimates of postnatal mental disorders are expressed in rates per 1000 deliveries
- delivery data are readily available to commissioners to support service planning.
This benchmark could also be applied to the number of live births, which some commissioners may choose to use to plan demand for services. The number of live births (rounded to the nearest 100) by primary care organisation can be obtained here.
Prevalence estimates from the psychiatric morbidity survey report (2001) have been used to estimate the proportion of women who may have mental disorders during pregnancy. The report gives the point prevalence estimates for the following mental disorders:
- mixed anxiety and depression
- depressive episode
- generalised anxiety disorder
- obsessive compulsive disorder
- panic disorder.
Mixed anxiety and depression is defined in the psychiatric morbidity survey report (2001) as a ‘catch-all' category that includes people with significant symptoms that cannot be coded into any of the other conditions included in the survey. We have assumed that women with post-traumatic stress disorder are counted within this group.
Applying the age- and sex-specific rates of the above disorders to the numbers of deliveries in each age band suggests that around 20% of deliveries are to women with one or more of the above conditions. This takes into account a possible overlap between the disorders covered by the psychiatric morbidity survey.
There is little evidence that the prognoses of disorders that develop during pregnancy or the postnatal period are significantly different from those that develop at other times. Similarly, there is little evidence that the underlying course of most pre-existing mental disorders is significantly altered during pregnancy and the postnatal period. Therefore, the estimate of 20% is thought to be an appropriate indication of mental disorders among women during pregnancy and the postnatal period because it reflects the prevalence of common mental disorders in the English population. The prevalence of depression alone in the postnatal period has been estimated to be around 10%.
Research suggests that in around 0.2% of deliveries, the woman may require an inpatient admission for a psychotic episode, and in a further 0.2% of deliveries the woman may need to be admitted for treatment for non-psychotic depression. Research also suggests that 0.2% of deliveries are to women with schizophrenia who will require advice and care from a specialist perinatal mental health service.
We have assumed that women admitted into secondary care for non-psychotic depression are counted within the 20% of deliveries estimated above.
IMS Disease Analyzer is a database that holds patient data from a sample of GP practice systems. Data were extracted on the basis of Read codes on a broad range of mental disorders including those not covered by the psychiatric morbidity survey, such as bipolar disorder and schizophrenia.
The validity of conclusions based on data extracted from IMS Disease Analyzer was assessed by comparing the rate of recorded births derived from the extracted data with the rate of deliveries among women in the general population. The two sets of data had comparable delivery rates per 1000 female population.
Analysis of the data suggests that:
- around 25% of women who were identified as having given birth had a diagnosis of one or more mental disorders of differing severities recorded on their medical records at any time in the past
- around 8% of women who were identified as having given birth had a diagnosis recorded on their medical records as having a mental disorder in the antenatal period
- around 14% of women who were identified as having given birth had a diagnosis recorded on their medical records as having a mental disorder in the postnatal period
- around 6% of women were identified as having both presented to their GP in the antenatal and postnatal period and having a diagnosis of a mental disorder recorded on their medical records.
This suggests that around 16% ([8% + 14%] - 6%) of women may require some form of intervention for a mental disorders during the antenatal and/or postnatal period. This estimate takes into account the overlap between women who present during pregnancy and those who present during the postnatal period.
The quality of data used in the analysis of diagnosed mental disorders relies on the information recorded within patients' medical records. In particular, poor detection and/or recording of mental disorders by healthcare professionals may lead to underestimation of the total numbers of women in contact with services who have a mental disorder.
Expert clinical opinion
The consensus opinion of the topic-specific advisory group was:
- Based on clinical practice and literature review, around 20%, or 200 per 1000, deliveries may be to women with a range of mental disorders with varying degrees of severity.
- Between 3% and 5% of deliveries are to women who have severe and/or complex mental disorders and may require advice and care from a specialist perinatal mental health service. A small proportion of these women may require admission to a mother and baby unit. The rate of admission per 1000 deliveries to a mother and baby unit is estimated to be:
- 2 per 1000 deliveries for psychosis
- 2 per 1000 deliveries for non-psychotic depression
- 2 per 1000 deliveries for chronic and complex mental disorders.
- Areas served by specialist community perinatal mental health outreach teams may be expected to have a lower rate of admission to a mother and baby unit of around 4 per 1000 deliveries. For the remaining 16% of deliveries, the proportion that are to women who may require and take up referral for psychological treatment is estimated to be around 50%. However, this is subject to a high degree of uncertainty and local variation.
Based on the epidemiological data and other information outlined above, it is concluded that 12% of deliveries or 120 per 1000 deliveries are to women who may require additional support and/or appropriate onward referral. This is based on the following assumptions:
- Around 20% of deliveries will be to women who experience mental disorders of varying degrees of severity. This is based on the prevalence of common mental disorders from the psychiatric morbidity survey report, analysis of data from IMS Disease Analyzer (accounting for a probable degree of under-recording and detection) and the expert opinion of the topic-specific advisory group.
- Around 4% of deliveries (midpoint of the estimates provided by the topic-specific advisory group) will be to women who have severe and/or complex mental disorders and are vulnerable to admission to secondary care. These women will require advice and care from a specialist perinatal mental health service. They include the estimated 4 per 1000 deliveries to women who will require admission to a mother and baby unit.
- Of the remaining 16% of deliveries, around half (or 8%) are to women who will require and take up the offer of psychological therapies.
Therefore the benchmark rates are estimated to be:
- 80 per 1000 deliveries (8%) to women who require referral for psychological therapies
- 40 per 1000 deliveries (4%) to women who require advice and care from a specialist perinatal mental health service; this includes 4 per 1000 deliveries to women who are likely to require admission to a mother and baby unit.
Use the antenatal and postnatal mental health service commissioning and benchmarking tool to determine the level of service that might be needed locally and to calculate the cost of commissioning the service using the indicative benchmark and/or your own local data.
1. Brockington I (1996) Motherhood and mental health. Oxford: Oxford University Press.
2. The Royal College of Psychiatrists (2001) CR88. Perinatal mental health services. Recommendations for provision of services for childbearing women. London: The Royal College of Psychiatrists
This page was last updated: 02 March 2012
- Antenatal and postnatal mental health services
- Commissioning antenatal and postnatal mental health services
- Specifying antenatal and postnatal mental health services
- Determining local service levels for antenatal and postnatal mental health services
- Assumptions used in estimating a population benchmark
- The commissioning and benchmarking tool
- Ensuring corporate and quality assurance