Sexual Health

Teenage Pregnancy

The new method of deriving information

Introduction

In previous years, there were substantial differences between Scotland and England & Wales in the ways in which teenage pregnancy rates are calculated.  These differences meant that the data were not directly comparable, although there was often an assumption by some users that published data from the countries of the UK could be compared.  In the past this has lead to misreporting and misinterpretation.
 
Background

Description of the old method of calculation of teenage pregnancy rates used by ISD

• Scottish data usually included miscarriages derived from SMR01, although a table excluding miscarriages had been produced in recent years.  Miscarriages increase the level of teenage pregnancies by approximately 6% for the 13-15 age group and approximately 8% for the 16-19 group (and the 13-19 group overall because the small proportion in the 13-15 group are swamped by the numbers in the older teenage group).  In recent years the proportion of miscarriages has decreased slightly.  This probably reflects an increasing preparedness to manage miscarriage without hospital admission.
  
• SMR02 (data returns from maternity hospitals) were used to derive the number of births and stillbirths.  This allowed the actual gestation at birth to be used, which is important since more than 10% of babies are born either three weeks or more before their due date or more than one week after it. But this advantage is offset by the fact that approximately 2% of births are not recorded on SMR02 and there have recently been substantial delays and incompleteness of SMR02 returns from certain areas.
  
• The abortions data were derived from SMR01 and SMR02 returns rather than notifications of legal abortions.
  
• The data were usually presented in specific age bands (13-15; 16-19, and 13-19), with both numerator and denominator within these bands.
  
• The data were usually presented by financial year rather than calendar year.
  
• The Scottish data were presented by date of the measured event rather than the date of the conception.  Thus a woman who conceived in 2003 and had her baby in 2004 would be included in the data for 2004 in Scotland, but 2003 in England & Wales.

Description of the method of calculation of teenage pregnancy rates used in England and Wales

• The source data are registrations of one or more live births and stillbirths (note that births of multiple babies should be counted as one event), and notifications of legal abortions.
  
• The date of conception for each pregnancy is calculated from the recorded gestation minus fourteen days for stillbirths and abortions.  The correction is because the period of gestation is traditionally measured from the first day of the last menstrual period, and it is assumed that conception starts two weeks after this date.  For live births, the date of conception is presumed to be 38 weeks before birth.
  
• The date of conception is used as the "event date" for the numerator.
  
• The geographical location of the woman at conception is approximated by using the postcode recorded at the time of the end of the pregnancy.  For abortion data, missing postcodes are imputed with a random postcode from within the main Primary Care Trusts (PCTs) of other residents attending the same hospital or clinic.
  
• For abortion data, the method of abortion is checked against the recorded gestation for compatibility.
  
• Where the gestation is not recorded on the abortion form, either 7,8,9 or 10 weeks is randomly assigned.  If the gestation is stated as three weeks, this is recoded to four weeks.
  
• If age is missing on the abortion forms, it is assigned to the 20-24 year group.
  
• If the gestation is more than 24 weeks, the "grounds" for abortion is checked.
  
• Confirmation of date of birth is sought for all women where the age is either under 15 or over 50.
  
• The data are usually presented as under 20, under 18 and under 16, but the denominators chosen for these rates are 15-19, 15-17 and 13-15 respectively.
  
• The denominators are ONS mid-year populations (based on updates of Census data).

The new approach

As far as possible, we have emulated the approach used in England & Wales.  Registration data are obtained from GROS for live and stillbirths.  These are processed to ensure that multiple births were treated as a single conception.  For all livebirths, the date of conception is calculated as being 38 weeks prior to the date of delivery.  For stillbirths, the recorded gestation in weeks is used, and two weeks are subtracted from this number to produce the estimated time between conception and birth.  This number is used to derive the likely conception date.  For the very small numbers of stillbirths in which the gestation is not recorded, 32 weeks is assumed.

The completeness and accuracy of the abortion data is assessed, and the gestation is used to calculate the date of conception.  For the small number of cases where gestation is missing, nine weeks is assumed.  Although this approach is not absolutely identical to the approach used in England and Wales it is considered to be as close as practicable and satisfactory for direct comparisons.

The numerators and denominators are derived in the same way as those used in England and Wales.

Comparison of old method and new method

Figure 1 Comparison of different methods of calculating teenage pregnancy

Figure 1 demonstrates that the different methods yield different pictures.  The traditional way of presenting these data seems to show a more marked fall in recent years.  This probably relates to incompleteness in the SMR02 returns. 

There is much similarity between the 13-15 and the under 16 data.  This is because the denominators are identical and the numerators are very similar although the under 16 data includes the very small number of women aged less than 13 but not the miscarriages.  Furthermore, because the line is fairly flat, there is little difference between the numbers when counted by year of conception and when counted by year of outcome.  Since abortion is a very common outcome in this age group, there is usually a short time lag between conception and outcome anyway.  The scale of the chart also tends to reduce the impact of any difference.

Similar charts have been produced for  NHS Board areas.

Comparison between Scotland and England & Wales

A direct comparison of teenage pregnancy rates in Scotland and those of England and Wales is illustrated in Figure 2.
 
Figure 2. Teenage pregnancy rates per 1,000 female population.  Direct comparison between Scottish data and England & Wales data.

This illustrates that for the under 20 age group, Scotland has a slightly lower teenage pregnancy rate than that of England & Wales.  However, the rate for Scotland is rising whereas that for England & Wales shows a slight fall between 2004 and 2005.  In the last year, the rates for the under 18 age groups have converged.  The figures for the under 16 group are very similar between the countries.

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