Postnatal heparin: what’s the evidence?

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?

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, heparin 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 heparin 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 heparin 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

Heparin 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 in this blog post, and the research focused on women who did not already have a blood clot.

Some people think that postnatal heparin is a beneficial intervention, and that’s partly because maternal mortality reports in the UK showed that some maternal deaths resulted from blood clots. Heparin started to be offered as a result, and the rate of fatal VTE went down.

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 side effects, which means we need to look carefully at the bigger picture, and ensure that women have good information on the pros AND the cons, so that they can make the decisions that are right for them.

But until now, we haven’t had any scientific evidence to tell us about the effectiveness of postnatal heparin when given on a population basis, so that’s why these research studies are so valuable.

The study

Lu et al (2021) carried out a study in the USA to see whether giving heparin 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 heparin was introduced, and how many women had a VTE after heparin 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, heparin 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 wrote 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.

This was confirmed in 2025, when researchers compared international clinical practice guidelines for postpartum venous thromboembolism prophylaxis.

Their study confirmed that:

“There are considerable differences in the rates of women receiving postpartum pharmacological venous thromboembolism prophylaxis when recommendations from different international guidelines are applied. These differences reflect the wide variation in guideline recommendations for the use of LMWH following birth.” (Ephraums et al 2025).

Quantifying risk

Kotaska explains, as I did above, that giving heparin 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).

A lot of harm!

The figures showing how many women are harmed in this area are really worrying.

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 heparin, 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.

A 2025 study

In 2025, another study on this topic was published, again based on data from the USA.

This time, researchers carried out a, “…multicenter retrospective cohort study of all individuals delivering at more than 20 weeks of gestation at four U.S. hospitals from 2016 to 2019. Individuals with a personal history of VTE or thrombophilia, with an antepartum diagnosis of VTE, or receiving therapeutic anticoagulation were excluded.” (Bruno et al 2025).

It is important to note that most of the women in this study received heparin only while they were in the hospital. This differs from what is offered in some other countries, where women are given heparin injections to take home and administer themselves. There was variation between the number of doses that were received. Most hospitals were giving the heparin every 12 hours, so those who stayed in hospital for several days would clearly have had more doses than those who left within a day or two.

It should be noted, however, that there isn’t robust evidence that any of the current inpatient or outpatient (at home) treatment protocols make a positive difference.

Indeed, the fact that there exist such a wide variety of approaches is itself evidence of the lack of consensus in this area.

Did heparin make a difference?

Bruno et al (2025) found that:

“Of 64,886 deliveries included, the rate of heparin-based postpartum pharmacologic thromboprophylaxis was 13.8% (95% CI, 13.5–14.1%), and the rate of VTE was 0.11% (95% CI, 0.09–0.14%).

Individuals receiving thromboprophylaxis were more likely to be older, to deliver by cesarean, and to have a comorbid health condition. In propensity score analysis, pharmacologic prophylaxis compared with no pharmacologic prophylaxis resulted in no difference in VTE (risk difference 0.0%, 95% CI, 0–0.16%) but an increased risk for hospital readmission (risk difference −1.36%, 95% CI, −2.51 to −0.14%) and wound complications (risk difference −1.45%, 95% CI, −2.35 to −0.65%).” (Bruno et al 2025).

I have put the key phrase in bold in the paragraph above, but let me spell it out for anyone who isn’t familiar with reading research:

Having postnatal heparin didn’t decrease the chance of having a venous thromboembolism.

In fact, those who had heparin were more likely to be readmitted to hospital, and the researchers noted that some women who had heparin after a caesarean were readmitted for wound complications that would probably not have occurred if they hadn’t been given the heparin.

Offering more light

Bruno et al (2025) offer more light on the history of this intervention, explaining that heparin-based prophylaxis has been shown to be beneficial in trials looking at patients who have undergone general surgery.

However, the patients in these trials have tended to be older, and anyone who is in need of general surgery is likely to be less healthy (and perhaps less mobile) overall than a young woman who is healthy, and having a surgical procedure for the purpose of giving birth rather than a surgical procedure to correct a health condition. 

It is particularly relevant that most women who give birth are likely to be fairly mobile relatively soon after surgery, not least because they have at least one child to care for (Bruno et al 2025).

What next?

The question now is to see whether the findings of these studies and the lack of evidence of benefit for postnatal heparin 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) noted, a trial to see whether aspirin is a better alternative to heparin is 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, my Plus Size Pregnancy book contains two sections explaining the research in this area in detail. Although it looks at heparin offered to those women with a higher BMI, much of what I’ve written on this topic in Plus Size Pregnancy applies more widely as well.

Plus Size Pregnancy

 

 

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