• Reference Number: HEY1660/2026
• Departments: Maternity Services
• Last Updated: 31 July 2026

What is Fondaparinux?

Fondaparinux is a synthetic blood-thinning injection that helps prevent or treat blood clots.

It contains no porcine (pig derived) products.

It works by blocking a clotting factor called Factor Xa.

Although not formally licensed for pregnancy, current evidence and expert guidance suggest it is safe when used under specialist advice.

Why am I being advised to take a blood thinner?

During pregnancy and after birth the risk of blood clots can be higher. Your team may recommend blood thinner injections if you have risk factors such as:

• Age, BMI and number of previous births
• Previous blood clots
• Surgery or caesarean birth
• Reduced mobility
• Certain medical conditions

Blood thinners help reduce this risk.

What are the options?

1. Low-molecular-weight heparin (LMWH) (e.g., enoxaparin)

• First-choice treatment in pregnancy for preventing and treating clots
• Widely used, safe and well-studied
• Contains pig (porcine) products

2. Fondaparinux

Used only when LMWH cannot be used, such as:

• Allergy or intolerance to LMWH
• Heparin-induced thrombocytopenia (HIT), A rare reaction that lowers the platelet count.
• You do not wish to use porcine products

Can be used for prevention or treatment of clots.

How safe is fondaparinux in pregnancy?

There is limited experience of its use in pregnancy, but evidence so far is reassuring:

• Used safely in pregnancy when LMWH not suitable
• Very limited placental transfer: no harmful effects reported in babies
• No dose changes usually needed
• Unknown if it enters breast milk, but it is unlikely to be taken in by your baby.

Experts consider it a reasonable alternative when LMWH cannot be used.

Possible side effects

Most people have no serious problems with fondaparinux.

If side effects happen, they are usually mild.

Common

Bruising, pain, or soreness where the injection is given, low blood count (which can make you feel tired), bleeding

Less common

Some people may have:

Chest pain or shortness of breath, fever, feeling sick or vomiting, swelling in the legs or body, skin rashes or itching, changes in blood tests (liver function and low platelets), wound discharge

Rare

Very rarely, people may feel:

Dizzy, tired, or sleepy, confused or anxious, headache, stomach problems (such as pain, constipation, or diarrhoea), low blood pressure, allergic reactions.

Get urgent help if you have:

• Heavy or unusual bleeding
• Chest pain
• Trouble breathing
• Fainting or collapse

Labour, birth and epidurals

Fondaparinux stays in the body longer than some other blood thinners.

• To have an epidural or spinal, enough time must have passed since your last dose:
  • At least 36 hours if taking fondaparinux to prevent clots
  • Up to 48 hours if taking a treatment dose for an active clot
• Do not take your next dose if you think labour has started or if you have any bleeding.
• Contact maternity triage or your midwife for advice.
• If labour starts earlier than planned, there is a higher risk of bleeding and an epidural or spinal may not be safe.

Your care team will assess you carefully and may involve specialist doctors to ensure your safety. In some cases, the higher bleeding risk may mean that blood transfusion or blood products are required. If an emergency caesarean section is needed, there is a higher chance of a general anaesthetic, meaning you would be asleep during the operation.

• If you have a planned induction or caesarean birth, your maternity team will give you a personalised plan and advise when to stop fondaparinux, usually 36 to 48 hours before birth.

Using fondaparinux after birth

• After the birth, your team will tell you when it is safe to restart fondaparinux, this is usually 6 to 12 hours after birth or after removal of the epidural and depends on how the birth went, whether you had an epidural or spinal, and your risk of bleeding.
• It is not known whether fondaparinux passes into breast milk, but even if a small amount does, it is unlikely to affect your baby, as it is broken down in the stomach.
• Low-molecular-weight heparin (LMWH) (e.g. enoxaparin) remains the preferred and best-studied option, but fondaparinux is a reasonable alternative when LMWH cannot be used.
• No harmful effects have been reported in breastfed babies whose mothers received fondaparinux.

Practical tips

• Inject once daily at the same time
• Rotate injection sites
• Report any unusual bleeding
• Follow the plan given by your maternity team

Should you require further advice on the issues contained in this leaflet, please do not hesitate to contact the Maternity Department.

General Advice and Consent

Most of your questions should have been answered by this leaflet, but remember that this is only a starting point for discussion with the healthcare team.

Consent to treatment

Before any doctor, nurse or therapist examines or treats you, they must seek your consent or permission. In order to make a decision, you need to have information from health professionals about the treatment or investigation which is being offered to you. You should always ask them more questions if you do not understand or if you want more information.

The information you receive should be about your condition, the alternatives available to you, and whether it carries risks as well as the benefits. What is important is that your consent is genuine or valid. That means:

• you must be able to give your consent
• you must be given enough information to enable you to make a decision
• you must be acting under your own free will and not under the strong influence of another person

Information about you

We collect and use your information to provide you with care and treatment. As part of your care, information about you will be shared between members of a healthcare team, some of whom you may not meet. Your information may also be used to help train staff, to check the quality of our care, to manage and plan the health service, and to help with research. Wherever possible we use anonymous data.

We may pass on relevant information to other health organisations that provide you with care. All information is treated as strictly confidential and is not given to anyone who does not need it. If you have any concerns please ask your doctor, or the person caring for you.

Under the General Data Protection Regulation and the Data Protection Act 2018 we are responsible for maintaining the confidentiality of any information we hold about you. For further information visit the following page: Confidential Information about You.

If you or your carer needs information about your health and wellbeing and about your care and treatment in a different format, such as large print, braille or audio, due to disability, impairment or sensory loss, please advise a member of staff and this can be arranged.