For many years the standard anti coagulants to treat and prevent deep vein thrombosis (DVT) have been heparin and low molecular weight heparionoids e.g Enoxaparin (Clexane®)  or alternately but generally less frequently Tinzaparin (Innohep®), Dalteparin (Fragmin®, Fondaparinux (Arixtra®). Alternatively warfarin may be given as a tablet but generally takes too long to stabilise the clotting mechanism. All of these drugs are compatible with breastfeeding. The sub cutaneous injections are too large molecules which cannot be absorbed from breastmilk (are not orally bio-available). Because of the very low milk levels with warfarin doses less than 12 mg daily, amounts ingested by the infant are small. No adverse reactions in breastfed infants have been reported from maternal warfarin use during lactation, even with a dose of 25 mg daily for 7 days. There is a consensus that maternal warfarin therapy during breastfeeding poses little risk to the breastfed infant. No special precautions are necessary. (Lactmed https://www.ncbi.nlm.nih.gov/books/NBK501137/).

See also Using heparins during breastfeeding Jan 2024 https://www.sps.nhs.uk/articles/using-heparins-during-breastfeeding/

Although these drugs are not absorbed from milk, caution is recommended where there is donation to a milk bank of surplus breastmilk. Should any small amount reach a pre-term baby it may result in a bleed with catastrophic results. There are no studies to confirm this but the recommendation is made to protect these most vulnerable babies.

Deep vein thrombosis

Whilst DVTs are uncommon they are 5 times more common in pregnancy and in the first 6 weeks after delivery than in women who are not pregnant.

The risk of developing DVT during pregnancy is even greater if :

• there is a history or family history of DVT
• age over 35
• obese BMI >30
• a severe infection or recent serious injury, which restrict exercise
• thrombophilia
• multiple birth
• after fertility treatment
• after a caesarean section
• currently smoke
• have severe varicose veins
• dehydration

https://www.nhs.uk/pregnancy/related-conditions/complications/deep-vein-thrombosis/

If the clot breaks off into the bloodstream, it can block a blood vessel in the lungs. This is called a pulmonary embolism (PE) and needs emergency treatment. It may be investigated by a VQ or CT scan (preferably the latter as it involves no interruption of breastfeeding . See https://breastfeeding-and-medication.co.uk/fact-sheet/breastfeeding-after-vq-scans

The newer treatments to prevent clots are called Direct Oral Anticoagulant Medications (DOAC) which have advantages in that they are oral medications and need less frequent blood tests to confirm the INR.

UKDILAS have produced an excellent summary https://www.sps.nhs.uk/articles/using-oral-anticoagulants-in-breastfeeding-women/

Rivaroxaban  (Xarelto®): compatible with breastfeeding

• Doses from breastmilk are over 100 times lower than doses given directly to pediatric patients. (Hale).
• Several case reports consistently indicate that maternal doses of rivaroxaban of 15 to 30 mg daily produce low levels in milk that are considerably below doses required for anticoagulation in infants (Lactmed https://www.ncbi.nlm.nih.gov/books/NBK500742/.
• Its pharmacokinetic data (large volume of distribution and high percentage of protein binding) explain the very small passage to milk observed in four different cases (Zhao 2020, Muysson 2019, Saito 2019, Wiesen 2016). https://www.e-lactancia.org/breastfeeding/rivaroxaban/product/
• See also Potential treatment option of rivaroxaban for breastfeeding women: A case series 2024. https://www.sciencedirect.com/science/article/abs/pii/S0049384824001166
• This is one of the preferred choice DOACs. Milk levels are low. Although rivaroxaban has a high oral bioavailability, very low levels are expected in milk due to its other pharmacokinetic properties, so infant exposure should still be minimal. https://www.sps.nhs.uk/articles/using-oral-anticoagulants-during-breastfeeding/

Dabigatran (Pradaxa ®) compatible with breastfeeding

• There are no studies on the transfer of dabigatran to breast milk. The molecular weight is large (628 Daltons) and the oral bioavailability is low (6.5%); therefore, clinically relevant levels are not expected to occur in a breastfed infant. (Hale).
• In adults, less than 7% of dabigatran is absorbed orally in its prodrug form of dabigatran etexilate mesylate; dabigatran itself is not absorbed orally. Preliminary data from 2 individuals indicate that dabigatran is poorly excreted into breastmilk and unlikely to affect the breastfed infant. If the mother requires dabigatran, it is not a reason to discontinue breastfeeding. Because data are limited, monitor preterm or newborn infants for signs of bleeding. (Lactmed https://www.ncbi.nlm.nih.gov/books/NBK500744/
• Its low oral bioavailability (EMA 2018, Blech 2008) hinders transfer to infant plasma via breastmilk, except in premature infants and the immediate neonatal period when there may be increased intestinal permeability.It is the oral anticoagulant with the lowest excretion in breast milk (Daei 2021). https://www.e-lactancia.org/breastfeeding/dabigatran-etexilate/product/
• This is one of the preferred choice DOACs. Milk levels are likely to be low. Dabigatran etexilate is one of the largest of the DOAC molecules and has a large volume of distribution, therefore it would be expected to pass into breast milk in low amounts. Milk levels were tested from two breastfeeding women who took dabigatran etexilate 220mg as a single dose. The relative infant dose was calculated to be between 0.01 and 0.07%. The infants did not receive any breast milk during this time. Infant absorption is likely to be low. Dabigatran etexilate also has very low oral bioavailability, so the infant is unlikely to absorb clinically significant amounts from the breast milk. It is therefore very unlikely that the infant would get any side-effects.https://www.sps.nhs.uk/articles/using-oral-anticoagulants-during-breastfeeding/

Apixaban(Eliquis®) and Edoxaban (Lixiana®) due to limited research should not be prescribed during breastfeeding.