Antiphospholipid Syndrome (APS): Causes, Symptoms, Diagnosis and Treatment Options

What Is APS?

APS is an autoimmune disorder in which your body produces antibodies that attack proteins attached to phospholipids, the fats found in the membranes of your cells. Normally, your immune system leaves these proteins alone. In APS, it does not do so, leaving the blood-clotting system chronically overactivated and making clots more likely to form where they should not.

The placenta depends on a network of tiny blood vessels to deliver oxygen and nutrients to the developing baby. When abnormal clotting disrupts that network, the pregnancy can fail, sometimes very early, sometimes later. APS also appears to interfere directly with how an embryo implants in the uterine lining, which is why it shows up in women who have had multiple failed IVF cycles, not just those who have had miscarriages.

While APL is present in around 1–5% of healthy individuals, only a small proportion develop APS. When the syndrome is present, however, it is one of the most important treatable causes of recurrent pregnancy loss (RPL) and pregnancy complications.

The difficult thing about APS is that it often gives no warning before a pregnancy. You may not have any symptoms, a history of blood clots, or a known autoimmune condition. The first sign is often the loss itself, which is why understanding and testing for APS is important.

What Are the Types of APS?

APS is classified based on whether another autoimmune disease is present alongside it. The distinction affects how the condition is managed.

Primary APS

Primary APS means the condition exists on its own, without any associated autoimmune disease. You have no lupus or rheumatoid arthritis, just the antibodies and their effects. This is the more common presentation, and it is entirely possible to have it with no symptoms until a pregnancy loss makes it visible.

Secondary APS

Secondary APS occurs alongside another autoimmune condition, most often systemic lupus erythematosus (SLE). Approximately 30-40% of people with SLE have aPL, although only a proportion develop APS. Women with lupus are routinely screened for APS because the combination is common and carries a higher risk of both clotting events and pregnancy complications. Managing the underlying condition is part of the treatment plan in these cases.

What Causes APS?

The precise trigger is not fully understood, but the core problem is that your immune system starts producing antibodies that target phospholipid-binding proteins, mainly beta-2 glycoprotein I and prothrombin, on cell membranes. When these antibodies attach to those proteins, they disrupt the body's natural anticoagulant mechanisms, making blood more prone to clotting.

Several factors are associated with developing APS:

• Autoimmune diseases: SLE, rheumatoid arthritis and Sjögren's syndrome are the most commonly associated conditions.
• Infections: Certain infections, including HIV, hepatitis C and syphilis, can trigger transient antiphospholipid antibody production. These antibodies usually resolve once the infection clears, though they may persist in some people.
• Medications: Drug-induced antiphospholipid antibodies are most commonly associated with chlorpromazine, procainamide, phenytoin and quinidine.
• Genetic predisposition: APS tends to run in families. Certain gene variants appear in studies as potential susceptibility markers.
• Sex and hormonal factors: APS is significantly more common in women of reproductive age. Oestrogen may influence antibody production, which is also why oestrogen-containing contraceptives can raise clotting risk in women who test positive.

What Are the Symptoms of APS?

Many people with APS have no symptoms until a clotting event or pregnancy loss occurs. When symptoms do appear, they depend on where in the body the clotting is happening, and they can range from visible skin changes to serious vascular events.

General symptoms related to blood clots include:

• Deep vein thrombosis (DVT): A clot in the deep veins of the leg, usually causing pain, swelling and warmth. If the clot travels to the lungs, it forms a pulmonary embolism, a medical emergency that requires immediate attention.
• Stroke or transient ischaemic attack (TIA): Clots in cerebral blood vessels can cause sudden facial drooping, arm weakness or difficulty speaking. Stroke in a young person with no obvious risk factors is a recognised presentation of APS.
• Heart valve abnormalities: Libman–Sacks endocarditis, a non-infectious inflammation of the heart valves, occurs in some people with APS.
• Livedo reticularis: A net-like purplish mottling of the skin caused by impaired blood flow in small skin vessels, often noticed on the legs.

Thesigns of APS in pregnancy most often include:

• Recurrent miscarriages: Three or more losses before 10 weeks, or one or more losses after 10 weeks, are a defining clinical criterion. Losses linked to APS often occur after a heartbeat has been seen, which makes them particularly hard to process.
• Late pregnancy loss: Loss of a morphologically normal foetus at or after 10 weeks of gestation.
• Severe, early-onset preeclampsia: High blood pressure and organ damage developing before 34 weeks, often more abrupt than typical preeclampsia.
• Preterm birth before 34 weeks: Often related to placental insufficiency or the need for early delivery due to preeclampsia.
• Placental abruption: Premature separation of the placenta from the uterine wall, sometimes associated with foetal distress or loss.

How Does APS Affect Fertility and Pregnancy?

If you have had repeated miscarriages or failed IVF cycles, APS may be a factor, and understanding how it works can make treatment feel less daunting.

How Does APS Cause Miscarriage and RPL?

APS causes pregnancy loss through two mechanisms. The first is thrombosis: clots form in the small blood vessels supplying the placenta, cutting off oxygen and nutrients to the developing baby. The second is more direct: APL appears to bind to proteins on the trophoblast, the cells that form the placenta and anchor it to the uterine lining. When this process is disrupted, the placenta does not embed properly from the very beginning of pregnancy.

APS accounts for approximately 10–15% of cases of RPL, making it one of the most common identifiable causes, and unlike chromosomal abnormalities, one that can be treated.

Can APS Cause Implantation Failure in IVF?

APS can also prevent a pregnancy from establishing at all, not just end one that has started. APL can bind to proteins on the surface of the endometrium and interfere with the molecular signals that allow an embryo to attach and embed. If you have transferred good-quality embryos in two or more IVF cycles without a resulting pregnancy, your fertility specialist may consider evaluating for APS, particularly if you also have RPL or other clinical features suggestive of the condition. When APS is identified, and treatment (usually aspirin and heparin) is started before a transfer cycle, outcomes improve significantly.

How Is APS Diagnosed?

Diagnosis requires two things: a relevant clinical history and confirmed positive antibody tests on at least two occasions. Having only one is not enough, and a single positive test is not sufficient on its own.

The internationally accepted standard is the Sapporo criteria (revised in Sydney in 2006). The blood tests for APS include:

Lupus Anticoagulant (LA) Test

Despite the name, this is not a check for lupus. It detects antibodies that interfere with phospholipid-dependent clotting reactions. Results can be affected by anticoagulant medications, so it must be done when you are not taking heparin or warfarin.

Anticardiolipin Antibody (aCL) Test

This measures IgG and IgM antibodies against cardiolipin. Medium to high titres, above 40 GPL or MPL units, are clinically significant. Low titres can appear transiently after infections and are not diagnostic on their own.

Anti-Beta-2 Glycoprotein I Antibody (Anti-β2GPI) Test

This measures antibodies against a protein that regulates coagulation. IgG anti-β2GPI antibodies carry the highest association with thrombosis and pregnancy complications.

What Are the Diagnostic Criteria for APS?

A positive antibody test alone does not diagnose APS. A confirmed diagnosis requires at least one clinical criterion and at least one laboratory criterion:

The 12-week gap between tests is clinically essential. A single positive result may be transient, following a viral infection, for instance, and does not constitute APS. You need persistent positivity on repeat testing before a diagnosis is confirmed.

What Are the Treatment Options for APS?

APS has effective treatments, and the difference they make to pregnancy outcomes is substantial. If a diagnosis feels overwhelming, it helps to know that this is one of the few causes of RPL with a clear, proven management plan.

Low-Dose Aspirin

Low-dose aspirin (75–100 mg daily) is the foundation of APS management in pregnancy. It reduces platelet stickiness, making the blood less likely to form small clots in placental vessels. Aspirin is often started before conception to establish its effect in time, not at the first positive pregnancy test. For women who have had only early miscarriages and no thrombotic events, aspirin alone may be sufficient, though most specialists add heparin.

Low Molecular Weight Heparin (LMWH)

LMWH, such as enoxaparin, is added for women with a history of later pregnancy loss, placental complications or prior blood clots. It is given as a once-daily injection under the skin of the abdomen and does not cross the placenta, meaning it carries no direct risk to the baby.

Treatment continues throughout pregnancy and for six to twelve weeks after delivery, because the postpartum period carries its own elevated clotting risk. Studies show that the combination of aspirin and heparin can increase the live birth rate in women.

Managing Underlying Autoimmune Conditions

When APS occurs alongside SLE or another autoimmune disease, controlling that condition is part of the treatment plan. Hydroxychloroquine, widely used in lupus, has independently shown a protective effect in women with APS and is often continued through pregnancy. Outside of pregnancy, warfarin may be used for long-term clot prevention, but it is not safe during pregnancy because it crosses the placenta.

How Is Pregnancy Managed When You Have APS?

A pregnancy with APS is managed as high-risk from the start, not to alarm you, but because closer monitoring genuinely improves outcomes. The goal is to catch any complications early and act before they escalate.

Why Does Treatment Need to Start Before Conception?

Ideally, low-dose aspirin begins before conception, and LMWH starts as soon as a pregnancy is confirmed on a blood test, not when a scan confirms a heartbeat. This early start matters because the placenta begins forming in the first weeks of pregnancy, precisely when APS-related disruption is most damaging.

What Monitoring Is Done During Pregnancy?

Monitoring throughout pregnancy typically includes uterine artery Doppler scans at 20–24 weeks to assess placental blood flow, serial growth scans from 28 weeks onward, and regular blood pressure and urine protein checks to detect preeclampsia early. Anticoagulation is paused before delivery, ie, LMWH is stopped 24 hours before a planned induction or caesarean, and the timing is coordinated well in advance by your obstetric team.

How Is Pregnancy Managed When You Have APS?

A pregnancy with APS is managed as high-risk from the start, not to alarm you, but because closer monitoring genuinely improves outcomes. The goal is to catch any complications early and act before they escalate.

Why Does Treatment Need to Start Before Conception?

Ideally, low-dose aspirin begins before conception, and LMWH starts as soon as a pregnancy is confirmed on a blood test, not when a scan confirms a heartbeat. This early start matters because the placenta begins forming in the first weeks of pregnancy, precisely when APS-related disruption is most damaging.

What Monitoring Is Done During Pregnancy?

Monitoring throughout pregnancy typically includes uterine artery Doppler scans at 20–24 weeks to assess placental blood flow, serial growth scans from 28 weeks onward, and regular blood pressure and urine protein checks to detect preeclampsia early. Anticoagulation is paused before delivery, ie, LMWH is stopped 24 hours before a planned induction or caesarean, and the timing is coordinated well in advance by your obstetric team.

What Are the Chances of a Successful Pregnancy With APS?

The statistics are genuinely encouraging once treatment is in place. Before effective treatment became standard, women with APS experienced significantly higher rates of pregnancy loss. Today, studies show that 70–80% of women can have a successful pregnancy outcome with low-dose aspirin and heparin therapy. Women who test positive on all three antibody tests, called triple positivity, carry a higher individual risk and need more intensive follow-up, but even this group can achieve successful pregnancies with the right care.

The number of losses you have already had does not predict what happens next once APS is identified and properly treated. What matters most is having a team that understands the condition, including an obstetrician experienced in high-risk pregnancy and ideally a haematologist or rheumatologist involved from the preconception stage.

When Should You Get Tested for APS?

Testing is worth considering if you have experienced any of the following, and a positive result on one occasion must always be confirmed on a repeat test at least 12 weeks later.

• Two or more unexplained miscarriages.
• One or more pregnancy losses after 10 weeks, especially if the baby appeared structurally normal on scan
• Preterm birth before 34 weeks, particularly if driven by severe preeclampsia or poor foetal growth
• Two or more failed IVF transfers with good-quality embryos
• An unexplained blood clot: a DVT or pulmonary embolism in a young person with no obvious cause
• A diagnosis of SLE or another autoimmune condition
• A first-degree relative with APS, recurrent miscarriages or unexplained clotting

Testing is done when you are not pregnant and not on anticoagulants, as both can affect results.

Conclusion

Finding out you have APS can feel like two things at once: a difficult answer and a hopeful one. Difficult, because it means your immune system has been working against you in ways that were not visible. Hopeful, because APS is one of the few identifiable causes of RPL with a clear, effective treatment. With the right combination of aspirin, heparin, close monitoring, and specialist care, most women with APS do go on to have a baby. Getting tested is the first step, and it is a step worth taking.