Whats the first question that comes in mind when doctor prescribed you enoxaparin injection? Why? Already so painful? Do I actually need injection really now?
Why do we even need medicines like blood thinners in pregnancy?
Is it really that dangerous if we skip them?

Pregnancy is a natural process. So when a doctor suddenly talks about heparin injections or aspirin every day, it feels opposite of natural. Many women think, My mother never took this, why should I? or If baby is inside, why are you thinning my blood from outside?

The honest answer is: pregnancy changes how blood behaves. It makes blood a bit thicker and more ready to clot. For most women this is fine and safe. But in some women, especially those with past miscarriages, blood clotting tendency, pre-eclampsia risk, heart valves or previous big clot in leg or lung, this same natural change can becomes difficult.

Pregnancy itself makes blood a little more sticky. This is nature’s way to protect from bleeding at delivery. The same change also increases the risk of dangerous clots in legs and lungs. Guidelines from the researching bodies like Society for Maternal-Fetal Medicine and other bodies say the risk of serious blood clots in pregnancy and after delivery is about four to five times higher than in women who are not pregnant.

So in selected women, a blood thinner is not extra medicine. It actually becomes part of safe pregnancy care. That includes many fertility and IVF patients too.

Important: This article is for education only. It does not replace a personal consultation with your fertility specialist, obstetrician or hematologist.

Why pregnancy raises clot risk

During pregnancy, the body changes in three big ways:

Blood becomes more coagulable
The liver makes more clotting factors. Natural clot blockers go down. This shift prepares the body for blood loss at birth, but it also makes clots easier to form in deep veins and in the lungs.

Blood flow in legs slows
The growing uterus presses on pelvic veins. This slows return of blood from legs. Slow flow is another trigger for clot formation.

Vessel wall changes and lifestyle
Pregnancy hormones relax vessel walls. Many women also move less because of discomfort, nausea, bed rest or long travel. All this raises risk further.

Risk becomes highest in the weeks just after birth, especially after caesarean delivery, long labour, heavy bleeding, obesity or infection.

For most low risk women, body still handles this fine without any medicine. For some, it does not. That is where blood thinners come in.

What counts as a blood thinner in pregnancy

In pregnancy doctors mainly use two groups:

1. Heparin type medicines

• Low molecular weight heparin, like enoxaparin
• Unfractionated heparin

These medicines act on clotting proteins in blood. They reduce formation of harmful clots.

Key point for parents: studies and guidelines agree that these heparins do not cross the placenta in usual doses. So they do not thin the baby’s blood directly.

Because of this, low molecular weight heparin is the preferred treatment in pregnancy for women who need full clot treatment or prevention.

2. Low dose aspirin

Low dose aspirin (ecosprin) at 75 to 150 mg per day works more on platelets, not on the full clotting system. In pregnancy it has a special role:

• It improves blood flow in the placenta
• It lowers risk of preeclampsia and early birth in high risk women

PGIMER Chandigarh, American College of Obstetricians and Gynecologists and Society for Maternal-Fetal Medicine advise low dose aspirin for women at high risk of preeclampsia, starting between 12 and 28 weeks, best before 16 weeks, and continuing till delivery.

It is not the same as full blood thinner like heparin. But many patients and even doctors use the same word casually.

When doctors use blood thinners in pregnancy

Your doctor will do a risk assessment before suggesting heparin or aspirin. Major guidelines from PGIMER Chandigarh all say this.

Past serious clot in leg or lung

If a woman had a clot in a deep leg vein or a clot in the lung earlier in life, especially during a previous pregnancy or after a birth or surgery, her risk in a new pregnancy is high.

In such cases, guidelines usually recommend low molecular weight heparin right through pregnancy and for some weeks after birth.

This is not optional in such women. It is life saving.

Inherited clot tendency

Some women inherit clot problems such as factor V Leiden mutation, prothrombin gene mutation or strong deficiencies of natural clot blockers.

If this is combined with a past clot or strong family history, or with pregnancy loss, many experts advise low molecular weight heparin during pregnancy and after birth.

Antiphospholipid syndrome and pregnancy loss

Antiphospholipid syndrome is an autoimmune condition that raises clot risk and increases risk of repeated miscarriages, stillbirth and placenta problems.

Trials and guideline reviews show that a combination of low dose aspirin and heparin, started early in pregnancy, increases live birth rate compared with aspirin alone in these women.

For an IVF or fertility patient with this syndrome, this combination often becomes part of the protocol.

Mechanical heart valves

Women with artificial heart valves need strong anticoagulation to prevent valve clotting, which can be fatal.

Studies show a difficult trade off:

• Warfarin protects the valve well but carries higher risk for the baby.
• Heparin protects the baby better but may slightly increase risk for the mother if dose and monitoring are not perfect.

Because of this, major cardiology and hematology guidelines state that every such woman needs individual planning by a high risk pregnancy team, not a standard recipe.

Prevention of preeclampsia and placenta problems

For women at high risk of preeclampsia, low dose aspirin is now standard of care in many countries.

Systematic reviews and large trials show that aspirin started between 12 and 16 weeks reduces risk of preeclampsia, preterm birth and growth restriction in high risk groups.

This includes some IVF patients, especially those with twin pregnancy, chronic high blood pressure, kidney disease, autoimmune disease or history of early severe preeclampsia.

High risk according to formal score

PGIMER Chandigarh recommends that every pregnant woman in hospital should have a structured clot risk score. Women with high scores receive heparin for prevention during pregnancy, after birth, or both.

Is it safe for the baby

Heparin

Low molecular weight heparin and unfractionated heparin do not cross the placenta.

So they do not thin the baby’s blood.

Their benefit is indirect: they keep blood flowing properly in the mother and the placenta.

This is why PGIMER Chandigarh strongly recommend low molecular weight heparin over no treatment, and also over unfractionated heparin, for pregnant women with acute serious clots.

Low dose aspirin

Low dose aspirin at 75 to 150 mg daily, used in high risk women, reduces preeclampsia, early birth and growth restriction without a big rise in major bleeding when used correctly. This is supported by multiple systematic reviews including Natl J Physiol Pharm Pharmacol, 2022.

Because of this, it is avoided in most pregnant women, except in rare mechanical heart valve situations under a specialised team.

What are the risks for the mother

Blood thinners are not perfect. Possible problems include:

• Bruising where heparin is injected
• Nose or gum bleeding
• More bleeding at birth or with caesarean delivery
• Rare allergy or heparin induced low platelet count.

Based on Journal of Obstetrics and Gynecology of India Research both support the view that low molecular weight heparin is safer than older heparin, with lower risk of platelets dropping and bone loss, when used at recommended doses in pregnancy.

So modern guidelines prefer low molecular weight heparin in almost all pregnancy settings where a true anticoagulant is needed.

How treatment is planned in real life

For an IVF or natural conception patient, a good clinic will usually follow these steps:

Before or early in pregnancy

• Ask about past clots, miscarriages, stillbirth, preeclampsia, autoimmune disease and family history.
• Check body weight, age, smoking, long travel and immobilisation.
• Order blood tests when indicated: thrombophilia work up, antiphospholipid antibodies, liver and kidney function.

Decision on blood thinner

• Check if the woman patient meets criteria for low dose aspirin to prevent preeclampsia.

Monitoring during pregnancy

• Regular antenatal visits with blood pressure and urine checks.
• Blood count and kidney function for those on heparin.
• Growth scans for the baby when there is clotting or placenta risk.

Planning the birth

• For women on full dose heparin, the guidelines suggest a planned delivery with stopping heparin ahead of time to allow safer epidural and reduce bleeding risk.
• Women on aspirin usually stop it near term as per local protocol.

After delivery

• The first six weeks after birth carry the highest clot risk. Many high risk women remain on heparin during this period.

What experts and guidelines say:

To build trust, it helps to know what top specialists actually write in peer reviewed sources:

1. Dr Nilanchali Singh and colleagues, New Delhi (low molecular weight heparin in Indian pregnancies)

At a tertiary care centre in North India, Dr Nilanchali Singh and team reviewed pregnant women who received low molecular weight heparin for different high risk reasons. They found that treatment was generally safe, with good maternal and baby outcomes and no major pattern of birth defects related to the drug. This work strongly supports the idea that low molecular weight heparin is a practical and safe tool in Indian pregnancy practice when used correctly.

2. Dr Charanpreet Singh, PGIMER Chandigarh – Indian perspective on clot disorders in pregnancy

In a 2023 review on thromboembolic problems during pregnancy in resource constrained Indian settings, Dr Charanpreet Singh and co authors stressed that women with a past serious clot and no antiphospholipid antibody syndrome should receive preventive anticoagulation during pregnancy and for six weeks after delivery. They also highlight that risk scoring and use of low molecular weight heparin are often under used in India, and more awareness is needed among clinicians.

3. Dr Rashmi Singla and team – heparin in high risk pregnancies with comorbidities

In the National Journal of Physiology, Pharmacy and Pharmacology, Dr Rashmi Singla and colleagues studied women with pregnancies complicated by conditions that increase clot risk, who were given low molecular weight heparin. Their work showed better obstetric outcomes and acceptable safety, supporting routine consideration of low molecular weight heparin in selected high risk Indian women, rather than avoiding it out of fear.

4. Dr Minakshi Rohilla, PGIMER Chandigarh – antiphospholipid antibody syndrome in Indian women

Dr Minakshi Rohilla and her team reported five years of experience managing Indian women with antiphospholipid antibody syndrome at a large tertiary centre. With early diagnosis and a protocol that uses low dose aspirin and anticoagulation, live birth rates improved and serious complications reduced compared to historic expectations. This gives strong local support to the use of aspirin plus heparin in women with this autoimmune clotting problem.

5. Dr M R Dabade and colleagues, Maharashtra – aspirin plus heparin in recurrent pregnancy loss

From Ashwini Rural Medical College in Solapur, Dr Dabade and team followed women with recurrent pregnancy loss linked to antiphospholipid antibodies or high homocysteine levels. They used a combination of low dose aspirin and low molecular weight heparin through pregnancy. Their study reported higher live birth rates with this combination compared to aspirin alone, adding Indian data to the international evidence that combined therapy helps in this specific group.

6. Indian obstetric guidance

Federation of Obstetric and Gynaecological Societies of India documents on medical disorders in pregnancy include Indian adapted recommendations on who should get clot prevention in pregnancy and after delivery. These documents broadly accept Royal College of Obstetricians and Gynaecologists style risk scoring, but also point out Indian realities such as late booking, limited testing and resource constraints. They encourage clinicians to be more proactive in using low molecular weight heparin in clearly high risk women instead of avoiding it due to fear or cost.

Frequently Asked Questions

1. Are blood thinners during pregnancy always dangerous

No. In women with true high risk, not using them is more dangerous. For example, a past large leg clot or lung clot carries a real chance of recurrence in pregnancy. Heparin can prevent a life threatening event in such cases.

2. Will heparin harm my baby

Current evidence and large guidelines say low molecular weight heparin does not cross the placenta. It does not thin the baby’s blood directly. The main aim is to protect you and your placenta.

3. I am doing IVF, Should I take aspirin or heparin to improve success

Not every IVF patient needs a blood thinner. In some conditions like antiphospholipid syndrome, major thrombophilia or repeated pregnancy loss, aspirin and heparin can improve outcomes. For routine IVF with no such issues, evidence is mixed and guidelines do not support blanket use. Your fertility doctor and hematologist should decide together.

4. Can I stop blood thinner on my own if I feel nervous

You should never stop heparin or aspirin suddenly in pregnancy without speaking to your doctor. If you stop early, clot risk may jump, especially in the days after birth. Any fear or side effect should be discussed, not hidden.

5. Does low dose aspirin make me bleed heavily in delivery

Studies show a small increase in some bleeding measures, but not a massive jump in life threatening bleeding when the medicine is used in guideline doses in the right patients. Your doctor will still watch you closely at delivery and manage any bleeding early.

References for further reading

• Singh N et al. Safety and efficacy of low molecular weight heparin therapy during pregnancy. Journal of Obstetrics and Gynecology of India. https://pubmed.ncbi.nlm.nih.gov/24431682/
• Singh C et al. Management of thromboembolic disorders during pregnancy in resource constrained settings. Indian Journal of Medical Research, 2023. https://pmc.ncbi.nlm.nih.gov/articles/PMC10466491/
• Singla R, Chaavi C. Effectiveness of low molecular weight heparin in obstetric outcome of pregnancies having comorbidities with potential thrombotic risk. Natl J Physiol Pharm Pharmacol, 2022. https://www.njppp.com/index.php?fulltxt=101373&fulltxtj=28&fulltxtp=28-1647148992.pdf
• Rohilla M et al. Perinatal outcomes in Indian women with antiphospholipid antibody syndrome. Eur J Obstet Gynecol Reprod Biol X, 2024. https://pubmed.ncbi.nlm.nih.gov/39296875/
• Dabade MR et al. Role of low molecular weight heparin and low dose aspirin in recurrent pregnancy loss with antiphospholipid antibody or pre antiphospholipid antibody syndrome and hyperhomocysteinemia. https://journal.barpetaogs.co.in/pdf/08200.pdf