From MIDWIFERY MATTERS, Issue No.112 - Spring 2007
Tessa Dean
I finished my midwifery training last autumn; started my motherhood training last year and am currently retaking my pregnancy training. I have always been slightly sceptical of the sweeping dietary 'advice' given to pregnant women, particularly that concerning 'foods which should be avoided'. As a passionate scrambled-egg eater and someone who has only ever (knowingly) had food poisoning abroad, I have always felt that I need more information about the true risks of a home-made chocolate mousse before I abstain altogether.
This article will look specifically at the advice given to women about avoiding salmonella and listeria infections whilst pregnant.
The concept of 'informed choice' is now ingrained in the maternity services, in sentiment if not reality. With regard to a pregnant woman's diet however, the 'choice menu' has fewer options than McDonalds. Women 'SHOULD NOT' eat this or that or the other. Even the otherwise balanced Informed Choice leaflets (MIDIRS, 2005) and recent articles in the RCM Journal (Williamson, 2006) and the Practising Midwife (Ford, 2004) convey the same poorly-referenced messages. The professional version of the Informed Choice leaflet on Diet and Nutrition during Pregnancy has no references at all for any of the items on the foods to avoid list.
The consultant midwife at one of the hospitals where I trained called such dietary dogma: 'social control of women', a view which may seem extreme until you realise that the list of research into these infections in pregnancy is as short as the list of foods to avoid is long. You too may find it ridiculous that many women will avoid a grape and brie sandwich (or worse, eat one and feel guilty) at the start of their pregnancy and then will have a couple of shots of what is essentially heroin at the end of it (in labour)!
Merely providing women with a list of 'foods to avoid' also leaves many questions unanswered:
? Is there a greater risk at certain times in the pregnancy?
? Is an infection from these bugs dose-related? i.e. can you have a spoonful (or two or three or four..) of raw cake mixture and not worry?
? Does it matter if you are used to eating the foods which could be problematic? Anecdotally, French women don't usually bother to avoid soft cheeses in pregnancy because they are so accustomed to eating them (though they are also tested for listeria as part of routine antenatal care).
? (And one for those amongst you prone to guilt.) Is it possible to have had an infection and not realize it? I.e. could you harm your baby as a result of the consumption of several pieces of feta cheese without knowing you'd been ill? One of the tutors on my course told me that this was possible and I genuinely worried throughout my first pregnancy that I may have caused the fetus some brain damage due to a hastily chosen Greek salad (ahh, the responsibility of being pregnant).
Let's start with Salmonella (as they might say in a dodgy foreign caf‚.) Salmonella is the name of a large (and anti-social) family of bacteria which can be found in poultry, eggs, unprocessed milk, meat and water (so that's just about everywhere then). It causes the sort of illness typical of food poisoning: diarrhoea (or constipation), nausea and vomiting, stomach cramps, etc. Pregnant women are given advice on general food hygiene and, more specifically, are advised to avoid raw or undercooked eggs. This is said to reduce the risk of becoming ill with a salmonella infection.
Nowadays more than 80% of eggs sold in the UK come from chickens which have been immunized against salmonella (specifically salmonella enteriditis). This includes all supermarket eggs and also eggs from other commercial egg farms in the UK. (The situation with foreign eggs, especially those from Spain, is different - but these are mostly only used in catering). In 2003 the Food Standards Agency (FSA, 2004) tested a total of 28,500 eggs to discover the occurrence of salmonella. Guess how many eggs were contaminated? Several thousand? Several hundred? 50? 10? No, it was NONE! Not a single egg had salmonella inside. However for those of you who are fantastically cautious they did find some salmonella bacteria on nine of the shells. In other words, it is more important to wash your hands after cooking with eggs than it is to make sure you cook them properly. Even so, nine eggs out of 28,500 means about 1 in 3,000 chance of finding a bug in your blackcurrant sorbet. Now, I like my eggs an awful lot but even I would be hard pushed to eat 3,000 of them in a single pregnancy (about 11 per day.).
This is the curious part. The clear cut advice to avoid raw and undercooked eggs makes it seem very likely that contracting salmonella will seriously harm your baby. However a web trawl reveals this belief to be misplaced. 'Salmonella isn't likely to have a direct adverse effect on the baby, but it is best avoided' (Hicks, 2005, from the BBC Health website). 'Although unlikely to harm your baby, salmonella should be treated with caution (!)' according to The NHS Direct website (2006) (my exclamation mark). I wonder exactly how one should treat invisible bacteria - which probably aren't there anyway - cautiously. There is very little research or evidence on the harm that potentially could be caused to your baby by a serious infection. One piece of research (Benshushan et al, 2002) states that salmonella infection can cause stillbirth - but the research piece is about listeria and the statement is unreferenced.
Listeria is a different petri-dish of bacteria altogether. An infection with the bacteria listeria (most commonly listeria monocytogenes) causes 'listeriosis' which can be more serious than salmonella. You are likely to suffer from an assortment of the following: fever, chills, muscle aches and diarrhoea. According to the BBC website (Hicks, 2005) the infection can cause miscarriage, stillbirth or severe illness in the newborn baby. Listeria bacteria lives in soil and water and can be found in unpastuerised milk and cheeses, raw meat and processed meals and salads. The general advice such as that given in the NHS's free 'Pregnancy book' is to avoid all mould-ripened soft cheeses such as Camembert, Brie, Stilton and Danish Blue. What is unclear is whether these cheeses are acceptable if pasteurised (as are many English bries) or whether the method of production still puts them at risk of contamination. The American 'Centers for Disease Control and Prevention'(2005) state that cheeses are 'permitted' if they are pasteurised. This isn't made clear in any British advice. A blanket approval of hard cheeses is normally given, though fresh Parmesan is nearly always made from unpastuerised milk, so is it a goody or a baddy? Listeria bacteria are also killed by cooking, which is why the advice on how to avoid them concerns avoiding poorly cooked meat (beware of barbecues!), and badly reheated chilled foods. Women are also advised to avoid pâté‚ and packaged salads such as coleslaw. Listeria bacteria certainly seem more powerful than weedy salmonella. On a hospital scale of power, listeria is the consultant whilst salmonella seems to work as the maternity assistant.
The NHS Direct website (2006) admits that listeriosis is rare in this country. So rare in fact that there is very little research on the subject of listeriosis in pregnancy. The 41 articles written in the last 15 years listed by the MIDIRS search engine include many case histories (a sure sign that there aren't enough subjects to do any quantitative research) and mostly neonatal case reports. There are no pieces of controlled research listed on the Cochrane Database using 'pregnancy and listeria' as search terms and only 17 (out of just under 500,000) pieces which contain the word listeria and none of these concern pregnancy.
The Patient Plus website (Tidy, 2005) states that listeria affects 6-15 pregnant women per 100,000 (though it omits a research reference for the statistic). One study looked at 65,000 women over a 10-year period and found 11 cases of listeriosis (Benshushan et al., 2002). This resulted in: 2 late abortions, 4 caesarean sections for fetal distress, 4 premature labours and 1 neonatal death at term. This research is quoted as showing that 20% of women who become infected with listeria will suffer from late abortions (or stillbirths) which is a frightening statistic until you realize it only means two women out of 65,000. However this research took place in Israel, which may well have a different listeria risk to the UK. Calculator to the rescue, I found that these statistics equate to one case of listeriosis in between 6,000 and 17,000 pregnancies. If we consider that there is approximately a one in 10,000 chance of dying in childbirth, it makes soft cheese look a lot less frightening.
There has been a suggestion that the reduction in cell-mediated immunity during pregnancy makes women more susceptible to infections such as listeriosis. The American Centers for Disease Control and Prevention state that pregnant women are 20 times more likely to contract listeria than other members of the population. But guess what? This statement is unreferenced (but often quoted) so we can't be sure of its accuracy.
Of course, one could argue the fact that listeriosis is still very uncommon obviously means that the advice is working! However that ignores the thousands of women who are pregnant without realising it (which let's face it is everyone for a while.); the women who inadvertently eat something that they didn't realise was on the forbidden list; and those who carry on as usual because they can't believe something they've eaten every week for that last 20 years with never so much of a burp of indigestion is suddenly going to result in a nasty illness.
If you are pregnant and suddenly come down with a flu-like illness with a high temperature and you or your doctor is clever enough to guess that it might be listeriosis (several articles have stressed the importance of considering a listeria infection if the woman presents with such symptoms) then you can be treated with antibiotics and this is often successful. Apparently the fetus can even be given antibiotics in utero (Silver, 1998).
? Is there a greater risk at certain times in the pregnancy?
A The risks do not appear to vary throughout pregnancy, though the result of a bad infection with either group of bacteria will obviously vary according to the stage of pregnancy, i.e. a woman infected preterm could experience premature labour whereas a woman infected at term may need a caesarean section for fetal distress.
? Is an infection from these bugs dose-related?
A There appears to be some evidence of a dose-related risk of catching salmonella though obviously not from eating raw cake mixture! (So long as the eggs are from a reputable UK source.). Due to the shortage of research into listeriosis I can't find any evidence of a dose-related risk though intuitively there would be one.
? Does it matter if you are used to eating the foods which could be problematic?
A I couldn't find any evidence to support or refute the supposition that the familiarity one has with certain foods makes it less likely to suffer an infection. The nature of the immune system makes it seems likely that one would have some immunity to the bacteria if previously exposed.
? Is it possible to have had an infection and not realize it?
A It seems highly unlikely that an infection by either salmonella or listeria would pass unnoticed (see also below).
I would like to hear fewer 'shoulds' in the course of offering what is dubiously called 'advice' about diet. When it comes to home birth we seem unable to say: 'home birth is safer than hospital birth' and we hedge our bets with the woolly phrase: 'there is no evidence to suggest that hospitals are a safer place to give birth'. Equally, we seem unable to say: 'raw eggs are now safe to eat in pregnancy'. But really, how many tests on how many eggs need to be done? The Food Standards Agency found no salmonella bacteria inside nearly 29,000 eggs in 2003. There used to be a problem and the egg-producers have overcome it with poultry vaccination programmes. If you need a caveat it could be that the eggs need to come from the UK.
With regard to listeriosis, this is a very rare illness in this country. If a woman is particularly anxious, then the advice may be appropriate for her, as for other women, they might like to carry on as before and be reassured that the risks are very low. If they are still worried they can be reassured that cooking kills bacteria so they can have their stilton as a meal ingredient (properly) cooked.
In my opinion the most important point for women is that they need not worry if they have eaten anything that they later discover was one of the 'forbidden' foods. If they have not been ill then they have not been infected with either group of bacteria. These do not seem to be illnesses that can be caught without realizing it. You can reassure women that if they have enjoyed an extra large piece of ripe stilton on a digestive biscuit on Christmas Day (as I intend to) and were fine afterwards then their baby will not be born with two heads as a result.
Pregnancy can often be a period of anxiety - Arghh, will my body ever be the same again? - Crikey, is the baby okay? - Help, how on earth is it going to come out of me? - so much so that minimizing a woman's worries must be one of a midwife's priorities. If they can do this by being a little less frightening about the danger of certain foodstuffs this would be a start.
All this writing has made me hungry - can I get anyone some home-made (and guilt-free) cheesecake?
Tessa Dean qualified as a direct-entry midwife in September 2006. Thanks to the reduction of vacancies at the local hospital and the expansion of her abdomen due to baby no.2 (another homebirth in February, hopefully), she hasn't yet worked as a midwife.
Benshushan A, Tsfrir A, Arbel R, Rahav G, Ariel I and Rojansky N (2002). 'Listeria infection during pregnancy: A 10 year experience', Israel Medical Association Journal 4,10, 776-780.
Centers for Disease Control and Prevention (2005). Listeriosis. [www] available from: www.cdc.gov/ncidod/dbmd/diseaseinfo/listeriosis_g.htm accessed December 12th 2006.
Cochrane Database available from: www.thecochranelibrary.com
Food Standards Agency (2004) Report of the survey of Salmonella contamination of UK produced shell eggs on retail sale. [www] available from: www.food.gov.uk/multimedia/pdfs/fsis5004report.pdf accessed November 10th 2006.
Ford F (2004). 'A guide to nutrition in pregnancy', Practising Midwife, 7, 11, 24-26.
Hicks R (2005). Pregnancy nutrition- what to avoid. BBC Health [www] Available
from:www.bbc.co.uk/health/healthy_living/nutrition/life_pregnancy2.shtml accessed December 22nd 2006.
MIDIRS searches available from www.midirs.org.
MIDIRS (2005). The Informed Choice Initiative, Bristol.
NHS (2001). The Pregnancy Book, Health Promotion England, London.
NHS Direct (2006). Which foods should I avoid during pregnancy? [www] available from: www.nhsdirect.nhs.uk/articles/article.aspx?articleId=917
Silver HM (1998). 'Listeriosis during pregnancy', Obstetrical and Gynecological Survey, 53, 12, 737-740.ÿ
Tidy, C. (2005) Gastrointestinal infections in pregnancy. PatientPlus [www]
available from: www.patient.co.uk/showdoc/40002081/ accessed December 20th 2006.
Williamson C. (2006). 'Maternal nutrition guidance: Keeping the proportions', RCM Midwives 9, 9, 346-349.
This article was originally published in Midwifery Matters ISSUE 112, Spring 2007
AH updated 16 October 2008