Aspirin

(Date of issue: July 2014. Version: 2)

This factsheet has been written for members of the public by the UK Teratology Information Service (UKTIS). UKTIS is a not-for-profit organisation funded by Public Health England on behalf of UK Health Departments. UKTIS has been providing scientific information to health care providers since 1983 on the effects that medicines, recreational drugs and chemicals may have on the developing baby during pregnancy.

What is it?

Aspirin (ASA, Angettes 75®, Caprin®, Disprin®, Micropirin®, Nu-Seals®, Resprin®) is a ‘non-steroidal anti-inflammatory drug’ (NSAID).

Low dose aspirin (total dose of 75 to 300 milligrams per day) is sometimes used as a ‘blood thinner’ to help prevent heart attacks and strokes in individuals who have been identified by their doctor as being at high risk of these problems. Low dose aspirin is also often prescribed after week 12 of pregnancy in women at increased risk of pre-eclampsia, to reduce the chance of it occurring in their current pregnancy. Low dose aspirin is also occasionally used in women undergoing fertility treatment, and in early pregnancy in women who have had previous recurrent miscarriages, as some studies suggest that this improves the chance of a pregnancy continuing. 

Standard dose aspirin (up to 4 grams per day) is used to treat pain and fever.

This leaflet summarises the scientific studies relating to the effects of aspirin on a baby in the womb. It is advisable to consider this information before taking aspirin if you are pregnant.

Is it safe to take aspirin in pregnancy?

Low dose aspirin
There is no evidence that taking low dose aspirin in pregnancy will cause a problem to you or your baby if you are healthy and are not taking other medicines that are affected by aspirin. However, you should only take low dose aspirin during pregnancy if your doctor has advised you to do so.

Standard dose aspirin
Use of standard dose aspirin after 30 weeks of pregnancy is not advised. For most women, paracetamol is the recommended medicine to control pain or fever during pregnancy. If paracetamol does not control your pain it is important that you ask your doctor for advice before taking standard dose aspirin or any other NSAID at any stage of pregnancy.

What if I have already taken aspirin during pregnancy?

If you have taken or are taking any medicines, it is always a good idea to let your doctor know that you are pregnant so that you can decide together whether you still need the medicines that you are on and to make sure that you are taking the lowest dose that works.

Low dose aspirin
Use of low dose aspirin at any stage of pregnancy has not been associated with harmful effects.

Standard dose aspirin
If you have taken standard dose aspirin after week 30 of pregnancy, particularly for an extended period, it is important that you let your doctor or midwife know straight away as your baby’s wellbeing in the womb may need to be assessed. 

Can taking aspirin in pregnancy cause miscarriage?

Low dose aspirin
Three large studies that each combined the data from a number of smaller studies, all showed that there was no increased risk of miscarriage in women who took low dose aspirin during pregnancy.

Six studies have investigated whether women taking low dose aspirin during early pregnancy were less likely to have a miscarriage. While three of these studies showed that women taking low dose aspirin had a reduced risk of miscarriage, the other three showed that taking low dose aspirin did not make any difference to miscarriage rates.

Standard dose aspirin
No studies have specifically analysed whether pregnant women taking standard dose aspirin are at increased risk of miscarriage, and more research on this subject is required.

Can taking aspirin in pregnancy cause my baby to be born with birth defects?

A baby’s body and most internal organs are formed during the first 12 weeks of pregnancy. It is mainly during this time that some medicines are known to cause birth defects.

A number of studies have suggested possible links between taking aspirin in early pregnancy and various specific birth defects (malformations). In some cases, other studies investigating the same birth defect have shown no link. These studies generally did not analyse women who were taking low dose aspirin separately from women who were taking standard doses for pain relief.  The studies are detailed below: 

• Nine studies collected information from mothers whose babies had been born with gastroschisis (where part of the bowel protrudes outside of the body through a hole next to the tummy button) to find out which medicines they had taken in early pregnancy. While six studies showed a possible link between gastroschisis and taking aspirin, the other three studies did not. More research on this subject is required

• One study showed a possible link between aspirin use in early pregnancy and having a baby with a heart defect, however four further studies have not agreed with this finding.

• Two studies suggested a possible link between taking aspirin in early pregnancy and having a baby with a cleft lip and/or palate, but another two studies found no link.

• One study showed a link between aspirin use in early pregnancy and undescended testes in baby boys (where the testes have not come down into the sac), but another two studies have not agreed with this finding.

• Four studies have investigated links between taking aspirin in early pregnancy and having a baby with a neural tube defect (an opening of the spine or head e.g. spina bifida, anencephaly). One study identified a possible link between aspirin use in pregnancy and anencephaly (where the baby’s brain and skull do not develop properly). Another study identified a possible link with any type of neural tube defect, however, the remaining two studies found no link.

• One study showed a link between aspirin use in early pregnancy and malformations of the kidney, but one further study did not find a link between aspirin use in pregnancy and certain specific kidney malformations.

There is no proof that taking aspirin during early pregnancy causes any of the defects listed above, although there are now a number of studies showing a possible link to gastroschisis. However, none of these studies is perfect and more information, especially on the use of standard doses of aspirin in early pregnancy needs to be collected and analysed.

Can taking aspirin in pregnancy cause stillbirth?

Low dose aspirin
There is no evidence from the four studies that have investigated this, to show that women who take low dose aspirin during pregnancy are at increased risk of stillbirth.

Standard dose aspirin
No studies have specifically analysed whether pregnant women taking standard dose aspirin are at increased risk of stillbirth, and more research on this subject is required.

Can taking aspirin in pregnancy cause preterm birth?

Low dose aspirin
There is no evidence that use of low dose aspirin in pregnancy increases the risk of preterm birth. One large study analysed the data from 22 smaller studies of women who were at increased risk of pregnancy complications.  Women in this high risk group who were treated with low dose aspirin during pregnancy were less likely to have a preterm birth (before 37 weeks of pregnancy) than high risk women who did not take low dose aspirin. These results are only relevant to women with certain health conditions.  It is not advisable to take low aspirin in pregnancy unless your doctor has recommended it. 

Standard dose aspirin
No studies have specifically analysed whether pregnant women taking standard dose aspirin are at increased risk of preterm birth, and more research on this subject is required.

Can taking aspirin in pregnancy cause my baby to be small at birth (low birth weight)?

Low dose aspirin
The large study described above also analysed information on birth weight from 12 studies of women at increased risk of pregnancy complications. Taking low dose aspirin during pregnancy did not change the likelihood of these women having a baby with a low birth weight (<2500g).

Another large study showed that women who took aspirin during pregnancy were not more likely to have a low birth weight baby. However, in this study the dose of aspirin that the women were taking was not specified and was likely to been a mixture of both low and standard doses.

Standard dose aspirin
No studies have specifically analysed whether taking standard dose aspirin in pregnancy increases the chance of having a low birth weight baby.

What problems can taking standard dose aspirin after 30 weeks of pregnancy cause in my baby?

Before birth

Premature closure of the ductus arteriosus
Before birth, a blood vessel in the baby called the ductus arteriosus needs to stay open to supply the baby in the womb with nutrients and oxygen from the mother. The ductus arteriosus closes just after birth to allow the baby’s blood to flow through its lungs once it starts breathing.  “Premature closure of the ductus arteriosus” is the medical term used to describe this blood vessel closing before it is supposed to.

Taking standard dose aspirin after 30 weeks of pregnancy may cause the ductus arteriosus to close while the baby is still in the womb, and the baby may then need to be delivered early. It is important to contact your doctor or midwife straight away if you have taken standard dose aspirin after 30 weeks of pregnancy, especially if you have taken several doses or more than the recommended dose.

Taking low dose aspirin during pregnancy is not thought to increase the risk of premature closure of the ductus arteriosus.

Oligohydramnios (reduced fluid around the baby)
Use of NSAIDs (including standard dose aspirin) after 30 weeks of pregnancy may cause oligohydramnios, (where there is too little amniotic fluid in the sac around the baby). Oligohydramnios can result in:

• Poor development of the baby’s lungs (because inhaling amniotic fluid while in the womb helps to expand and develop the lungs).

• The baby’s bladder being under-developed (because urine expands and develops the bladder).

• The baby’s skull and leg bones being misshapen and the baby having ‘flattened’ facial features (as a result of there not being enough amniotic fluid to ‘cushion’ the baby in the womb).

• Compression (squashing) of the umbilical cord, which may result in reduced blood flow from the mother to the baby in the womb.

If you have taken standard dose aspirin after 30 weeks of pregnancy your doctor may wish to check your amniotic fluid levels by ultrasound scan.

There is no evidence that taking low dose aspirin during pregnancy is linked to reduced levels of amniotic fluid.

After birth

Persistent pulmonary hypertension of the newborn (PPHN)
PPHN occurs when a newborn baby’s lungs do not adapt to breathing outside the womb. PPHN only affects around 1 or 2 out of every 1,000 newborn babies in the general population, but can be serious. PPHN is thought to be linked, in some cases, to premature closure of the ductus arteriosus (see section above) and has also been linked to taking NSAIDs, including aspirin, around the time of delivery.

Blood clotting problems in the baby
Because aspirin ‘thins’ the blood, there have been concerns that babies exposed to aspirin while in the womb may have problems with blood clotting after birth.

Standard dose aspirin
One small study showed a possible link between blood clotting abnormalities in full-term babies and exposure to standard dose aspirin in the week before birth.

Low dose aspirin
One large study that combined from the information collected in twelve smaller studies found no link between taking low dose aspirin during pregnancy and having a baby with blood clotting problems.

Wherever possible, use of standard dose aspirin (e.g. to treat pain or fever) during the third trimester of pregnancy should be avoided. Please discuss any concerns that you may have with your doctor.

Can taking aspirin in pregnancy cause learning or behavioural problems in the child?

A baby’s brain continues to develop right up until the end of pregnancy. It is therefore possible that taking certain medicines at any stage of pregnancy could have a lasting effect on a child’s learning or behaviour. Most of the studies which have been carried out to investigate whether aspirin use in pregnancy is linked to learning or behavioural problems in the child are reassuring.

• One study showed that 18 month old children of women who had taken low dose aspirin during pregnancy were no more likely to have problems with movement and language than children whose mothers had not taken aspirin in pregnancy.

• Another study of children up to age 5 years who had been exposed to low dose aspirin in the womb showed no difference in the likelihood of behavioural problems, or problems of the nervous system, compared to children not exposed to aspirin

• Two studies have investigated the IQ (intelligence) of children born to women who took unknown doses of aspirin during pregnancy. While one showed that four year olds who had been exposed to aspirin in the womb were more likely to have a lower IQ than those that hadn’t, the other study showed that five year olds who had been exposed to aspirin in the womb were more likely to have a higher IQ

Studying whether children who have been exposed to a particular medicine are more likely to have problems with their learning or behaviour is complicated. Different tests are often used by different researchers, and developmental problems may only show themselves in later childhood. More research into any potential effects of aspirin use during pregnancy on the learning and development of the child is therefore required.

Will my baby need extra monitoring during pregnancy?

Most women will be offered a scan at around 20 weeks of pregnancy to look for birth defects as part of their routine antenatal care. Taking aspirin before 30 weeks of pregnancy would not normally require extra monitoring of your baby.

If you have taken standard dose aspirin after 30 weeks of pregnancy you may need extra scans or monitoring of your baby. 

Taking low dose aspirin at any stage of pregnancy does not cause problems that would require extra monitoring. However, some of the conditions for which low dose aspirin is prescribed (e.g. pre-eclampsia, recurrent miscarriage, heart disease in the mother) may themselves be a reason for women taking low dose aspirin to be more closely monitored during pregnancy.

Are there any risks to my baby if the father has taken aspirin?

We would not expect any increased risk to your baby if the father took aspirin before or around the time your baby was conceived.

Who can I talk to if I have questions?

If you have any questions regarding the information in this leaflet please discuss them with your health care provider. They can access more detailed medical and scientific information from www.uktis.org.  

General information 

Up to 1 out of every 5 pregnancies ends in a miscarriage, and 1 in 40 babies are born with a birth defect. These are referred to as the background population risks.  They describe the chance of these events happening for any pregnancy before taking factors such as the mother’s health during pregnancy, her lifestyle, medicines she takes and the genetic make up of her and the baby’s father into account.

Medicines use in pregnancy

Most medicines used by the mother will cross the placenta and reach the baby. Sometimes this may have beneficial effects for the baby.  There are, however, some medicines that can harm a baby’s normal development.  How a medicine affects a baby may depend on the stage of pregnancy when the medicine is taken. If you are on regular medication you should discuss these effects with your doctor/health care team before becoming pregnant.

If a new medicine is suggested for you during pregnancy, please ensure the doctor or health care professional treating you is aware of your pregnancy.

When deciding whether or not to use a medicine in pregnancy you need to weigh up how the medicine might improve your and/or your unborn baby’s health against any possible problems that the drug may cause. Our bumps leaflets are written to provide you with a summary of what is known about use of a specific medicine in pregnancy so that you can decide together with your health care provider what is best for you and your baby.   

Every pregnancy is unique. The decision to start, stop, continue or change a prescribed medicine before or during pregnancy should be made in consultation with your health care provider. It is very helpful if you can record all your medication taken in pregnancy in your hand held maternity records.

   

www.medicinesinpregnancy.org

Disclaimer: This information is not intended to replace the individual care and advice of your health care provider. New information is continually becoming available. Whilst every effort will be made to ensure that this information is accurate and up to date at the time of publication, we cannot cover every eventuality and the information providers cannot be held responsible for any adverse outcomes following decisions made on the basis of this information. We strongly advise that printouts should NOT be kept for any length of time, or for “future reference” as they can rapidly become out of date.

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