Antiphospholipid Syndrome (APS)

Antiphospholipid Syndrome (APS) is an immune disorder that makes the blood clot too easily and damages blood flow through the placenta. In APS, the immune system makes antibodies against proteins that bind to fats in the blood. These antibodies raise the risk of blood clots. They also harm the small vessels that feed the placenta. The three main antibodies are lupus anticoagulant, anticardiolipin antibodies, and anti-beta-2 glycoprotein I antibodies.¹

A diagnosis of APS requires two findings. One is a clinical event. The other is a lab result. Most clinicians still use the 2006 Sydney criteria in daily practice. Under these criteria, the clinical event is either a blood clot or one of three set patterns in pregnancy. The first is three or more unexplained losses in a row before ten weeks. The second is one or more unexplained losses of a normally formed fetus at or after ten weeks. The third is one or more preterm births before thirty-four weeks. The cause must be eclampsia, severe preeclampsia, or recognized signs that the placenta is failing. A preterm birth on its own does not meet this criterion. The medical reason for the birth must be recorded.² The lab criteria require at least one of the three antibodies. The test must be positive twice, at least twelve weeks apart. One positive test does not prove the diagnosis.²

In 2023, the American College of Rheumatology and the European Alliance of Associations for Rheumatology published updated classification criteria. These criteria use a weighted point system. They also require a positive antibody test within three years of the clinical event.³ Both the 2023 criteria and the 2006 Sydney criteria are in active clinical use.

In pregnancy, clots in the placenta cut the supply of food and oxygen to the growing baby. For this reason, APS can cause repeated pregnancy loss, poor growth of the baby, and loss later in pregnancy. APS is one of the most clearly defined and most treatable immune causes of recurrent pregnancy loss.

Blood thinners taken during pregnancy reduce this clotting injury to the placenta. Major reproductive medicine guidelines support blood thinner therapy (anticoagulation) for APS in pregnancy.¹ The treating clinician matches the plan to each woman's history and her antibody pattern.

Women with APS who have lost a pregnancy do not have unexplained loss. They have a named condition that can be treated. A test for APS belongs in a thorough recurrent pregnancy loss workup. Couples deserve that answer before anyone calls the loss a mystery. Related conditions include autoimmune and thrombophilic disorders, inherited thrombophilias, and natural killer cell dysregulation.