Hormones and breastfeeding

Behind the Headlines

Friday January 8 2010

A scientist has claimed that “breast milk is no more beneficial for a child's health than formula milk”, The Daily Telegraph reported. It said the researcher, Professor Sven Carlsen, said the child’s health is actually determined by the hormone balance in the mother's womb, with a high level of male hormones affecting both her ability to breastfeed and the baby’s health.

However, this study did not compare levels of male hormones during pregnancy to the baby’s health, but only to whether the mother breastfed after giving birth. The researchers have supplied no evidence to back up their claim that there is no benefit from breast milk.

The finding that women are more likely to breastfeed if they have lower levels of male hormones during pregnancy warrants further investigation. But for women who can breastfeed, the advice that ‘breast is best’ stands. Breast milk is the most nutritious feed for the baby, protects them against infections and offers various benefits to the mother, including helping to lose the weight gained during pregnancy and reducing the risk of contracting certain types of cancer.

 

Where did the story come from?

The research was carried out by Professor Sven Magnus Carlsen and colleagues from Trondheim University. The study was funded by the central Norway regional health authority and published in the peer-reviewed medical journal Acta Obstetricia et Gynecologica Scandinavica.

This small cohort study looked at data from pregnant women, including their hormone levels, and compared it with their breastfeeding habits after giving birth.

The media has mainly focused on Carlsen’s claim that breastfeeding is no more beneficial to a baby’s health than formula milk. Although the press release for this study states that the researchers reviewed “more than 50 international studies about the relationship between breastfeeding and health”, the results from this research do not appear to have been published. Therefore, what evidence supports this claim is unclear.

The researcher also claims that breastfeeding is associated with a slight health benefit for babies, but it is not the milk itself that is responsible. He says that successful breastfeeding is actually a sign that a mother had the optimal level of hormones during pregnancy resulting in her body developing in such a way that it became better able to produce milk. Higher levels of male hormones, Carlsen says, hinder the development of glandular tissues that produce milk, making it harder or less likely for the mother to breastfeed.

However, this is currently only a theory. The research only looked at data from pregnant women, comparing it with their breastfeeding habits after giving birth. The researchers have not supplied any evidence from research into how male hormone levels in the womb affect the subsequent health of the baby.

 

What kind of research was this?

This small retrospective cohort study looked at data from pregnant women, including their hormone levels, and compared it with their breastfeeding habits after giving birth.

The study design - a cohort study - can only indicate what might influence breastfeeding, rather than show what actually causes a woman to decide whether to breastfeed or not. There are many physical, psychological, social and environmental reasons that can influence a woman’s decision, and this study was not set up to investigate all of these possible reasons.

 

What did the research involve?

The study included women who were part of a Norwegian study conducted between 1986 and 1988.

The study included women with singleton pregnancies (one foetus) and who had had one or two prior pregnancies. When they were enrolled, their age, BMI before pregnancy and smoking status at conception was recorded. Information on the women’s education was also collected.

Two groups were assessed - a random sample of 561 pregnant women and a second group of 1,384 women who were at risk of having low birthweight babies (high-risk group). These women had a higher risk because they had a history of a previous low birthweight babies, or their babies had died around the time of birth, were smokers when the baby was conceived, weighed less than 50kg before they were pregnant, or had chronic kidney disease or high blood pressure.

A stored frozen blood sample taken at 25 weeks of pregnancy and information on breastfeeding at six weeks, three and/or six months was obtained from 63 women in the random group and 118 women in the high-risk group. The researchers only analysed data from these small groups of women in this study. The babies’ feeding was classified as breastfeeding only, breastfeeding with supplementation and supplementation only.

The hormones dehydroepiandrosterone (DHEA), DHEAS, testosterone, androstenedione, and the sex hormone binding globulin (SHBG) were measured in the blood sample.

 

What were the basic results?

Both the group of randomly selected women and the high-risk group had similar hormone levels, maternal age, number of weeks gestation at delivery, infant birthweight, and breastfeeding and supplementary feeding. Women in the high-risk group had a lower average BMI than the random group.

The study assessed the likelihood of breastfeeding at six weeks, and three and six months. The women who smoked at the time of conception were less likely to be breastfeeding at three months. Women who had high levels of testosterone during pregnancy were also less likely to be breastfeeding at three or six months.

The likelihood of breastfeeding was not affected by maternal BMI, gestational age at birth, birthweight, or gender of the child.

In the high-risk group the likelihood of breastfeeding was lower at six weeks and three months with increased gestational dehydroepiandrosterone levels.

 

How did the researchers interpret the results?

The researchers suggest that breastfeeding (including women at high risk for low birthweight babies) was negatively associated with maternal androgen levels in the second trimester of pregnancy (about weeks 13-27). This means that the women were more likely to breastfeed and continue to breastfeed for longer if they had lower levels of male hormones in the second trimester of pregnancy.

They hypothesise that high androgen levels during pregnancy may affect the likelihood of breastfeeding by inhibiting transformation of the breast into the lactating state. They say that if male hormone levels are high during pregnancy they may also be high at birth. These hormones could either inhibit the production of milk, or have psychological effects on women, which may lower their dedication to breastfeeding.

 

Conclusion

This research indicates that there may be an association between male hormone levels during pregnancy and the likelihood of breastfeeding. However, it cannot say whether there is a causal relationship between male hormone levels and a decreased likelihood of breastfeeding. The study has some limitations, which the researchers acknowledge:

  • They highlight that the study was carried out on Caucasian women in a country where breastfeeding rates were high. The sample may not therefore be comparable to other populations in countries where breastfeeding is not as common.
  • The researchers did not have information as to why women did not breastfeed (whether they had difficulty doing so or chose not to). As there are many physical, psychological, social and environmental reasons as to why a woman may not breastfeed, it is not possible to speculate how male hormones affect this.
  • The research conducted multiple statistical analyses investigating associations between breastfeeding and different potential risk factors. A study that makes multiple comparisons like this raises the likelihood that its findings are due to chance rather than showing a true association.

Based on this one small cohort study, the claims that breast milk is of no benefit to health are unfounded. The study did not investigate whether breastfed babies were healthier or not, but looked only at levels of their mother’s hormones during pregnancy and their breastfeeding after giving birth.

This finding regarding male hormone levels during pregnancy and breastfeeding warrants further investigation. But for women who can breastfeed, the advice that ‘breast is best’ stands. Breast milk is the most nutritious feed for the baby, protects them against infections and offers various benefits to the mother, including helping to lose the weight gained during pregnancy and reducing the risk of contracting certain types of cancer.

Links to the headlines

Hormones 'govern ability to breastfeed'. BBC News, January 7 2010

Breastfeeding ‘not always best’. Daily Express, January 7 2010

Is breast really best? Professor defies official advice to hail formula milk as just as good. Daily Mail, January 7 2010

Breastmilk no better for baby than formula, scientist claims. The Daily Telegraph, January 7 2010

Breast “not best”. Daily Mirror, January 7 2010

Links to the science

Carlsen SM, Jacobsen G, Vanky E. Mid-pregnancy androgen levels are negatively associated with breastfeeding. Acta Obstetricia et Gynecologica Scandinavica 2010; 89: 87-94


 

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Analysis by Bazian

Edited by NHS Choices