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Indian Journal of Community Medicine

Estimating the Duration of Full Breastfeeding with Survival Analysis Procedure

Author(s): G Singh

Vol. 32, No. 1 (2007-01 - 2007-03)

Abstract

Background: Full breastfeeding refers to exclusive breastfeeding, which is the practice of feeding only breast milk excluding water, breast milk substitutes, other liquids and solid foods. `Full breastfeeding for the first six months is the most appropriate infant feeding practice. The duration of full breastfeeding varies considerably among women belonging to different sections of the population.
Objective: To estimate the probabilities at various time intervals for the duration of full breastfeeding.
Study Design: A group of 102 women who were interviewed at immunization centre in Varanasi in 2004, where they came with their youngest child for vaccination. Women reporting continuance of full breastfeeding (31 %) on the date of interview were considered as censored cases. Survival analysis procedure using life table approach was applied.
Result: The median duration of full breastfeeding was found 5.53 months: Relationship between duration of full breastfeeding and practice of breastfeeding (at fixed time or on demand) was also studied. The mean and median duration of full breastfeeding were observed higher in women feeding at fixed time than those on demand of the child, though by log-rank test the difference was found not statistically significant.
Conclusions: The observed data do not provide sufficient evidence to establish a relationship between duration of full breastfeeding and practice of breastfeeding.

Keywords: Exclusive Breastfeeding, Survival Probability, Median Duration

Breastfeeding provides required nutrients for proper growth and development of a Child, develops immunity to combat infections, promotes survival and provides some protection to mother against early conception. All women should practice exclusive breastfeeding from birth upto 4 to 6 months; thereafter children should continue to be breastfed while receiving appropriate and adequate complementary food for upto two years of age and beyond1. An early introduction of breast milk substitutes and late introduction of semi-solid complementary items are responsible for rapid increase in the prevalence of under-nutrition during 6 to 24 months of age2. WHO defines exclusive breastfeeding as the practice of feeding only breast milk (including expressed breast milk) and allows the baby to receive vitamins, minerals or medicines. Water, breast-milk substitutes, other liquids and solid foods are excluded. WHO World Health Assembly in 2001 resolved that exclusive breastfeeding for the first six months is the most appropriate infant feeding practice3. Earlier the ideal duration universally accepted for full breastfeeding was 4 to 6 months. Recent developments suggest full breastfeeding should continue to six months4,5. There is good evidence that two more months of exclusive breastfeeding (i.e. from fourth to sixth months) provides infants with additional protection against gastrointestinal infections during that two months period.

The objective of this paper was to estimate the probability of period of full breastfeeding since birth at various time intervals for a group of women incorporating some censored cases through survival analysis procedure. Censored cases are cases for which the second event is not recorded or simply does not occur before the end of the study. The first event in this context is the time after birth of a child when breastfeeding starts and second event refers to the end of full breastfeeding when some food supplements are introduced and breastfeeding becomes partial if continues. Thus the women reporting continuance of full breastfeeding on the date of interview were considered as censored cases. Presence of censored data makes the conventional statistical techniques inappropriate. A statistical technique useful for such situation is survival analysis, which is a method of estimating time-to-event models in the presence of censored cases. The term full breastfeeding used here refers to exclusive breastfeeding. As the study is based on interview of mothers, there remain some elements of subjectivity, recall lapses and digit preference. Therefore it might have happened for some women reporting on their duration and quality of full breastfeeding not to be much accurate or exclusive.

The aim of this paper was to analyse the observed data on duration of full breastfeeding of a group of women who had delivered a child within last one year from survey data. In other words subjects of the study were women whose youngest child is less than or equal to one year on the date of survey. Survival analysis procedure using life table method was applied and conditional probabilities of survival (continuance of full breastfeeding) at monthly interval of time were obtained. Standard error, hazard rate, and median duration of full breastfeeding were determined. The relationship between duration of full breastfeeding and practice of feeding at fixed time or on demand of the child was also studied and their survival prospects were compared using log-rank test. It is to be noted that survival at time t (or in an interval (t1, t2) here refers to the continuance of full breastfeeding at time t (or in the interval (t1, t2).

A sample of 102 women was selected from child immunization centre at Varanasi in the year 2004 where they came with their child for vaccination. Sample size was calculated on the basis of descriptive outcome measures considering the estimates of similar previous studies. Women included in the study were those whose youngest child was aged less than or equal to one year on the date of interview. Thus open birth interval of these women was at the most one year. Child immunization starts at birth and at regular interval different vaccines, its subsequent doses are recommended. Some children enter late in the immunization schedule and some do not observe perfect timing of the required dose. Therefore it may be assumed that mothers visit immunization centre throughout the year, not necessarily at some fixed time point. Thus the sampled subjects were mothers of children aged 1,2,3…12 months.

Inclusion criteria:

  • women aged 20-34 years,
  • parity of any order,
  • visiting the immunization centre with youngest child for vaccination,
  • youngest child aged less than or equal to 12 months.

Total 102 women thus selected were interviewed through a study proforma which included general information, sociodemographic status, marital duration, number of conceptions, number of birth, birth intervals, date of birth of youngest child, period of post-partum amenorrhoea associated with youngest child, duration of full breastfeeding, pattern of breastfeeding like full, partial, at fixed time or on demand. The duration of full breastfeeding. (in months) here refers to the period which begins when the first breast milk is given to the child and ends at the time when full breastfeeding terminates with the introduction of semi-solid or liquid food supplements alongwith breastfeeding.

Material and Methods

Women under study were asked about their duration of full breastfeeding in months related with the youngest child. Women who were continuing full breastfeeding on the date of interview were considered censored cases and their duration of full breastfeeding (since first breast milk given till the survey date) recorded and treated as censored data. For such women it was not known when would they discontinue exclusive breastfeeding and introduce other semi-solid or liquid food supplements alongwith breastfeeding. Subjects with censored data contribute valuable information and they should not be omitted from the analysis. It would also be wrong to treat the observed time at censoring as the survival time. Analysis of survival data requires special techniques because for some cases the observation period was cutoff before the event occurred. A statistical technique useful for this data is survival analysis. The basic idea of life table approach in survival analysis is to subdivide the period of observation into smaller time intervals and for each interval, all who have been observed atleast that long are used to calculate the probability of a terminal event occurring in that interval. The probabilities estimated from each of the intervals are then used to estimate the over all probability of the event occurring at different time points. There are three assumptions for this methodology6. First, at any time subjects who are censored have the same survival prospects as those who continue to be followed. Second, survival probabilities are the same for subjects recruited early and late in the study. Third, the event happens at the time specified. In this paper, life table method has been applied. Survival probabilities, standard error, hazard rate and median survival time have been obtained. Log-rank test was applied to compare the survival experiences of two groups formed on the basis of breastfeeding practices.

Results

The data collected were analysed using SPSS ver.10 statistical software and results shown in the following tables. Table 1 shows sample size (n=102), age range of women (20-34 years), parity, number of women who experienced the event (70), number of cases censored (32). A large proportion of women were housewives belonging to middle class family and coming from urban area.

Table 1: Baseline characteristics of study subjects

Total number of women, (n) 102
Age Range (years) 20-34
Parity ≥1
No. of women for whom event of interest occurred (full breastfeeding terminated on
or before survey date)
70   (68.6%)
No. of censored cases (continuing full
breastfeeding on the survey date)
32   (31.4/0)

The duration of full breastfeeding was divided into monthly time interval (Table 2, Column-I). Number of women who were fully breastfeeding their child at the start of a given time interval was shown in Col.-3 of Table 2. Number of women who terminated full breastfeeding in a given interval was shown in Col.4 (Table 2) and number continuing full breastfeeding in that interval was given in Co1.5 (Table 2). It can be seen from Table 2 that 17 women terminated full breastfeeding during 4-5 months interval while 6 women continuing full breastfeeding during this interval were censored. Maximum number of women (27) terminating full breastfeeding was observed in 5 to 6 months duration.

The median duration of full breastfeeding for the observed data was obtained as 5.53 months.

For each time interval Col.6 of Table 2 shows the probability that those who survived (full breastfeeding) at the beginning of a given time interval will survive to the end of that interval. The probability of survival at the end of an interval is shown in Col.7 of Table 2. Survival to any time point is calculated as the product of the conditional probabilities of surviving each time interval. In interval 4 to 5 months the value in Col.7 (0.70) is the probability that a woman will survive (remain in full breastfeeding) by the end of this time interval. The value in this column can be obtained from Col.6 (as for the interval 4 to 5 months: 0.7703 × 0.9209 × 0.9898 × 1.0000 × 1.0000 = 0.70). Thus Col.6 and Col.7 are important indicators of survival prospects of women. The survival probability is gradually decreasing with the increase in the time interval of full breastfeeding (Fig. 1). This decline is comparatively maximum between 4-5 months to 5-6 months interval. Hazard rate shown in Col.9 of Table 2 is the probability per unit time that a woman who is full breastfeeding at the beginning of an interval will experience the terminal event (i.e. discontinue full breastfeeding) in that interval. Hazard rate shows increasing trend upto interval 6-7 months where it attains maximum then downs in 7-8 months period.

In order to study the duration of full breastfeeding with respect to practice of breastfeeding, total 102 women were divided into two groups, one with those reporting feeding at fixed time and other consisting of those feeding on demand of the child. Survival curves for the two groups are shown in Fig. 2. Table 2 shows 57% women reporting fixed time and 43% on demand. Mean and median duration of full breastfeeding were observed higher (5.39 and 5.50 months respectively) for women breastfeeding their child at fixed time, than those who fed on demand (5.08 and 5.00 months respectively). However, the log-rank test for testing the equality of survival prospects (duration of full breastfeeding) of the two groups resulted not statistically significant (logrank= 1.48, p=0.2245).

Table 2: Impact of breast feeding on child survival

(1)
Full
Breastfeeding
Interval
(Months)
(2)
Interval
Start
Time
(3)
No.
Entering
this
Interval
(4)
No. of
Terminal
Events
(5)
No. of
Censored
Cases
(6)
Proportion
Surviving
(7)
Survival
Probability
at the end
of Interval
(8)
S.E. of
Survival
Probability
(9)
Hazard
Rate
0-1 0 102 0 0 1.0000 1.0000 0.0000 0.0000
1-2 1 102 0 0 1.0000 1:0000 0.0000 0.0000
2-3 2 102 1 8 0.9898 0.9898 0.0102 0.0103
3-4 3 93 7 9 0.9209 0.9115 0.0299 0.0824
4-5 4 77 17 6 0.7703 0.7021 0.0502 0.2595
5-6 5 54 27 8 0.4600 0.3230 0.0546 0.7397
6-7 6 19 14 1 0.2432 0.0786 0.0348 1.2174
7 – 8 7 4 2 0 0.5000 0.0393 0.0263 0.6667
8-9 8 2 2 0 0.0000 0.0000 0.0000 2.0000

Footnote:
1. The median survival time for these data is 5.53 months.
2. Col.-4 shows no. of women who terminated full breastfeeding in the given interval.
3. Col.-5 shows no. of censored women continuing full breastfeeding at the end of given interval. 4. Col.-6 shows probability of surviving in the interval if surviving at the beginning of the interval. 5. Col.-7 shows survival probability at the end of the given interval.

Table 3: Statistical Significance of Survival Analysis

Full
Breastfeeding
Number
(n=102)
Number
Events
Number
Censored
Survival
time (Months)
Log-Rank Test
for equality
of Practice
Survival
Distributions
Mean SE
95%C.I.
Median
SE 95%C.I
At Fixed Time 58
(56.9%)
33 25 5.39
±0.20
(4. 99, 5.78)
5.50
± 0.20
(5.07. 5.93)
Log-Rank = 1.48
d.f. = 1
p = 0.2245
On Demand 44
37 7 5.08
± 0.19
(4.71, 5.44)
5.00
±0.11
(4.78, 5.22)
 

Fig. 1: Survival Curve Showing Survival Probability for Duration of full Breastfeeding.

Fig. 2: Survival Curve Showing Survival Probability for Duration of full Breastfeeding at Fixed Time and on Demand. The Upper Curve is of Fixed Time Full Breastfeeding Women and Lower Curve is of Women Full Breastfeeding on Demand.

Discussion

In the present study when data on 102 women were analysed through survival analysis procedure, the median duration of full breastfeeding was 5.3 months. The data from National Family Health Survey-2 shows that in India, the median duration of full breastfeeding is about 5.3 months and median duration of total breastfeeding is 25.4 months. The duration of breastfeeding was shorter in urban educated women from high income group and those who had delivered in health facility(9). Thus the estimate obtained in present study is comparable with the national figure reported by NFHS-2. However, in a study in Bangladesh(10), the median duration of full breastfeeding was obtained as 3.67 months, which is below the estimate obtained here, the reasons for which should be worked out.

To determine whether the duration of full breastfeeding is related with the practice of breastfeeding (at fixed time or on demand), the survival experiences of the two groups were compared. The mean and median duration of full breastfeeding were observed relatively higher for women feeding at fixed time than those feeding on demand however, the difference was not statistically significant. Therefore, it may be concluded that the observed data do not provide sufficient evidence to establish a relationship between duration of full breastfeeding and practice of breastfeeding.

References

  1. Innocenti Declaration on Protection, Promotion and Support of Breastfeeding. 1 Aug. 1990, Florence, Italy.
  2. Ramachandran, P. Breastfeeding practices in South Asia. Indian Journal of Medical Research, 2004; 119 (6): 13-15.
  3. WHO. Infant and young child nutrition resolution 54.2 of the World Health Assembly. Resolutions and Decisions of 54t’’ World Health Assembly, 14-22 May 2001.
  4. Boland, M. Exclusive breastfeeding should continue to six months. Paediatrics and Child Health, 2005; 10 (3): 148.
  5. WHO. The optimal duration of exclusive breastfeeding. Report of an expert consultation. Geneva, 28-31 March 2001.
  6. Bland, JM and Altman DG. Survival Probabilities. British Medical Journal, 1998; 317:1572-80.
  7. Singh, G. Regression analysis of the effect of full and partial lactation on post-partum amenorrhoea in lower middle class Indian females. Indian Journal of Medical Sciences, 1997; 51(6): 181-5.
  8. Singh KK, Suchindran CM, Singh RS. Smoothed breastfeeding durations and waiting time to conception. Soc. Biol. 1994; 41(3-4): 229-39.
  9. National Family Health Survey-2. International Institute for Population Sciences, Mumbai. 2000.
  10. Giashuddin MS, Kabir M. Duration of breastfeeding in Bangladesh. Indian Journal of Medical Research, 2004; 119: 267-72.

Deptt. of Community Medicine, Ganga Bagh Colony, Lanka, Varanasi-221005 (U.P.)
E-mail: [email protected]
Received: 12.11.05

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