Postnatal heparin: what’s the evidence?

What’s the evidence for giving postnatal heparin?

For a few years now, some women have been offered a drug called low molecular weight heparin during and/or after pregnancy.

This is a preventative or prophylactic measure, given in the hope of preventing blood clots during pregnancy and/or after birth.

In simple terms, these drugs thin your blood and thus make blood clots less likely. Blood clots can occasionally be fatal, so that’s why this is an important thing to think about. But, like all drugs, heparin has side effects and downsides. And whenever we try to prevent something with a drug or intervention, we need to make sure that the benefits outweigh the downsides. That’s especially the case with women who are at a low risk of problems in the first place.


What is low molecular weight heparin?

Women with a higher BMI are very likely to be offered heparin, so there are two sections on this in Sara’s ‘Plus Size Pregnancy’ book.

The phrase ‘low molecular weight heparin’ is often abbreviated to LMWH. It’s also known by a number of other names, including Dalteparin (Fragmin), Tinzaparin (Innohep), and Enoxaparin (Clexane, Inhixa).

In many areas, LMWH is offered to women who have risk factors for blood clots. Risk factors include having had a blood clot in the past, having had surgery, being a smoker, being older or having a higher BMI, having had several babies before, and having varicose veins. The list is actually quite long and some risk factors are more important than others. In many areas, women are assessed against a list of risk factors. They are offered LMWH if they have one of the more significant risk factors which puts them in a “high risk” group, but even low risk women are offered LMWH in some areas if they have three or four of the ‘lesser’ risk factors. That’s explained further in the RCOG Green-top guideline on reducing the risk of venous thromboembolism. Venous thromboembolism is the medical name for the type of blood clot that we are concerned about, and it is often abbreviated to VTE.


Why we need research on postnatal heparin

LMWH can be offered in pregnancy and/or after birth. Postnatal (after birth) heparin is the subject of the study and commentary that I’m sharing today, and the research focused on women who did not already have a blood clot.

We’ve been thinking that postnatal heparin is a beneficial intervention, and that’s partly because some data suggested that the rate of fatal VTE went down in the UK after we began to offer LMWH more frequently. But just because two things happen in the same time period, they are not necessarily related. This is what we mean when we say that association is not causation. Sometimes, increased awareness of a problem like VTEs can make more of a difference than a drug given to try and prevent them. And all drugs have 

But until now, we haven’t had any scientific evidence to tell us whether LMWH works or not, so that’s why this research and the related commentary are so valuable.


The study

Lu et al (2021) carried out a study in the USA to see whether giving LMWH made a difference to the number of blood clots experienced by women. They didn’t include women who already had a blood clot, and that’s what they mean by “general population” in the quote below.

The researchers designed a retrospective cohort study, in which they looked at the medical records of “all patients who delivered at our tertiary care center from 2013 to 2018. Deliveries were categorized as preprotocol (2013–2015; no standardized heparin-based thromboprophylaxis) and postprotocol (2016–2018).” Lu et al (2021)

In simple terms, they compared how many women had a VTE before LMWH was introduced, and how many women had a VTE after LMWH was introduced.


And their findings?

“Risk-stratified heparin-based thromboprophylaxis in a general obstetric population was associated with increased wound and bleeding complications without a complementary decrease in postpartum VTE. Guidelines recommending this strategy should be reconsidered.” Lu et al (2021)

In other words, LMWH didn’t reduce the number of VTEs, but it did cause quite a lot of women to have bleeding complications. Which isn’t surprising, because heparin thins the blood. But, as a related commentary by Andrew Kotaska shows, the actual numbers in this area are quite surprising.


Raising concerns

Obstetrician Andrew Kotaska has challenged a number of other obstetric practices, myths and poorly-designed studies. He has previously shared his concerns that we are giving postnatal LMWH too freely and without due consideration of its harms. And he has written an editorial about Lu et al‘s (2021) study.

He begins with a summary of the situation in practice:

“Worldwide, increasing numbers of postpartum women are receiving low-molecular-weight heparin for venous thromboembolism (VTE) prophylaxis. Five national guidelines recommend liberal low-molecular-weight heparin in a large proportion of birthing women, including most women delivering by cesarean.” Kotaska (2021).

However, as with so many other situations, there is wide variation in what you will experience, depending on where you live in the world.


Quantifying risk

Kotaska explains, as I did above, that giving LMWH is a simple and well-intentioned intervention, designed to reduce deep vein thrombosis and pulmonary embolism after birth. It’s well intentioned because, as you now know, these can sometimes be fatal. But, as is the case with so many of the routine interventions offered in and around pregnancy and childbirth, the risk factors which lead to treatment are quite common, the problem itself is rare and the preventative measures have side effects. Here’s Kotaska’s (2021) summary of a key aspect of Lu et al’s (2021) findings in this area.

“Given that 4% of women in this study experienced wound and bleeding complications from low-molecular-weight heparin after cesarean delivery, an estimated 16,000 to 40,000 women would be expected to be harmed for every pulmonary embolism death prevented.” (Kotaska 2021).

This is a really high number. Although some people have been concerned about the number of women who were being offered LMWH, we haven’t had evidence until now of the actual risks and benefits. The Lu et al (2021) study gives us data about the chance of side effects relative to any benefit of giving LMWH, which is less than many people had hoped.

It’s quite common in obstetrics to discover, when we take an evidence-based approach, that the interventions that we are being offered do more harm than good. That’s not because the problem (in this case VTE) isn’t a problem. It’s because the methods we use to try and predict problems are crude, and the drugs or treatments we’re offering to try and prevent the problem have side effects. In some cases, the downsides of the drug or intervention can be far-reaching and affect many times more women/babies than the original problem would have affected.


What next?

The question now is to see whether this will be taken into account in practice. Obstetrics isn’t known to be fast at adopting evidence-based practices, which is why people like myself share information directly with women and families, so that everyone can decide for themselves.

As Kotaska (2021) notes, a trial to see whether aspirin is a better alternative to LMWH is currently underway but we should not necessarily assume that this is a better option. Interventions need to be proven to be safe and effective before they are given to large numbers of people and their use embedded in guidelines. Let’s hope these papers help with that goal.


If you’d like to know more, I’ve written a book which helps women and families to better understand the different aspects of making birth-related decisions, so you can make the decision that’s right for you.

You can see the research study that I’m talking about here: Evaluation of a Risk-Stratified, Heparin-Based, Obstetric Thromboprophylaxis Protocol.
Andrew Kotaska’s editorial is here: Postpartum Heparin Thromboprophylaxis: More Harm Than Good.



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