APS is usually associated with recurrent miscarriage, but it can also cause other pregnancy complications.

Recurrent miscarriage means having three or more miscarriages in a row, and it affects about one in every hundred couples trying for a baby. At least 15% of recurrent miscarriages occur as a result of APS, and it is now recognised as the most common, treatable cause. With correct diagnosis and treatment, the pregnancy success rate has risen from 20% before 1990 to around 70-80% today.

As APS pregnancies are classed as high risk, it is best to try and find a specialised or early pregnancy unit where doctors have prior clinical experience.

Treatment will depend on individual medical history, test results and current circumstances. Generally, for women diagnosed with antiphospholipid syndrome, aspirin and heparin should be offered from a positive test until at least 34 weeks of gestation, following discussion of potential benefits versus risks. She will be treated daily with low dose aspirin (75mg-150mg) and daily heparin injections.

If a woman has been diagnosed with APS prior to becoming pregnant and is already being treated with warfarin, she will have to change over to daily heparin injections, ideally before the embryo is six weeks old, as warfarin is potentially harmful to the baby.

Some women with APS can have successful pregnancies without any treatment – the reasons for this are still not clear – but pregnancy loss and/or complications are often the first and, in some cases, the only sign of the condition in women and it is then known as Obstetric APS - OAPS.

Pregnancies can be affected in a number of ways:

Early pregnancy loss

The majority of miscarriages in women with APS occur at the early stages of pregnancy in the first 13 weeks.

The antiphospholipid antibodies (aPL) cause early miscarriages because they prevent the pregnancy from embedding properly in the womb, and inhibit the growth of the early foetal cells. Some women who have very early recurrent miscarriages can sometimes be labelled as infertile.

Sadly, losing a baby in early pregnancy is quite common, with about one in five of all pregnancies ending in miscarriage. As there can be many other possible causes for early miscarriage, women will not be tested for antiphospholipid antibodies (aPL) until they have had three miscarriages in a row.

However, the Royal College of Obstetricians and Gyneacologists revised their guidelines in 2023 and now state: "In this guideline, recurrent miscarriage has been defined as three or more first trimester miscarriages. However, clinicians are encouraged to use their clinical discretion to recommend extensive evaluation after two first trimester miscarriages, if there is a suspicion that the miscarriages are of pathological and not of sporadic nature."

APS Support UK hopes this will help stop a lot of anguish for women who suffer miscarriages and are then diagnosed with APS and recommend that you ask your GP to be tested after two miscarriages, rather than having to go through the agony of a third.

Late pregnancy loss

Losing a baby in the second and third trimesters (from 14 weeks until birth) is rare in most pregnancies, but is very strongly associated with APS, with a number of losses occurring between three and six months.

As other causes of pregnancy loss, such as chromosomal abnormalities, are much less common in the second and third trimesters, any woman with late pregnancy loss should be tested for antiphospholipid antibodies (aPL).

The antibodies act in a different way in later pregnancy loss than with early miscarriage, by causing clots in the small and delicate veins in the placenta. The placenta is then unable to supply the baby with nutrients and oxygen so the baby may stop growing and die.


Pre-eclampsia affects up to 10% of all pregnancies, but it is twice as likely to occur in women with APS, begins at an earlier stage and is more severe.

It is now thought that pre-eclampsia is caused by a lack of blood being supplied to the placenta. As antiphospholipid antibodies (aPL) can damage the placenta by causing clots in the small vessels, it is highly likely this is why it is so common among women with APS. Research into the possible link between pre-eclampsia, miscarriage and infertility is currently underway.

Pre-eclampsia is a potentially dangerous condition and can result in high blood pressure, fluid retention (oedema) and protein in the urine (proteinuria) for the mother. In the unborn baby, it can cause growth problems.

If pre-eclampsia is not treated, it can lead to serious complications including placental abruption. Severe pre-eclampsia will need to be managed in hospital with medication to lower the blood pressure and control the symptoms. The only way to cure pre-eclampsia is to deliver the baby so, in many cases, labour has to be induced for the mother’s safety and the baby is born prematurely.

For more information, please visit Action on Pre-eclampsia.

Intrauterine growth restriction

Intrauterine growth restriction (IUGR) refers to the reduced growth of a baby while in the womb. It is estimated to affect between 10-30% of babies born to mothers with APS.

Babies are diagnosed with IUGR if they weigh less than 90% of other babies at the same gestational age. There can be lots of reasons why a baby appears small but, in women with APS, the antiphospholipid antibodies damage the placenta and can prevent the baby growing to normal size, so the baby is small at birth.

In the management of pregnancy in APS, Doppler ultrasound scans will be carried out to discover if there is a fall in foetal blood supply. If there is a problem, the scan can help the specialists decide whether to move towards an earlier (possibly Caesarean) delivery.

In the management of pregnancy in APS, Doppler ultrasound scans will be carried out to discover if there is a fall in foetal blood supply. If there is a problem, the scan can help the specialists decide whether to move towards an earlier (possibly Caesarean) delivery.

Support for the future

As it is not currently possible to identify which women have OAPS as opposed to thrombotic APS, any woman who has been diagnosed is potentially at increased risk from blood clots and should be advised to continue taking 75mg aspirin daily as a precaution. Long term research is being carried out looking into this, but it will be some years before the results are available.

It can be devastating to miscarry one baby after another, and the experience can place enormous strain on you and your partner. The Miscarriage Association's website: provides a wealth of information on coping strategies, leaflets and helplines all designed to support your grieving process. Similarly, losing a baby later on in pregnancy can be shattering - SANDS, the Stillbirth and Neonatal Death charity provides support and information to anyone affected by this tragedy.