To find the answer to this I had to find out the prevention measures and management of Postpartum Haemorrhage (PPH).
Oxytocin is typically used right after the delivery of the baby to prevent PPH (Weeks, 2015). Misoprostol may be used in areas where oxytocin is not available. Nipple stimulation and breastfeeding triggers the release of natural oxytocin in the body, therefore it is thought that encouraging the baby to suckle soon after birth may reduce the risk of PPH for the mother (Abedi, et al. 2016). The review by Abedi et al. (2016) looking into this did not find enough good research to say whether or not nipple stimulation did reduce PPH. More research is needed to answer this question.
So with this in mind, it seems that Oxytocin is a big clue.
The physiological effects of Oxytocin include uterine contraction. This is important for cervical dilation before birth, and oxytocin causes contractions during the second and third stages of labour. This also serves to assist the uterus in clotting the placental attachment point postpartum.
The answer to my question is that contraction of the uterine muscles during labour compresses the blood vessels and reduces flow, thereby increasing the likelihood of coagulation and preventing haemorrhage (Carroll, 2007).
At parturition, only 0.5 L [just over 1 US Pint or 0.88 UK Pints] of maternal blood is lost, with the remaining excess volume lost gradually (Carroll, 2007).
If the placenta (or part of it) is still attached or detached but still in the womb, there is no way the uterus can contract enough to compress the blood vessels.
A lack of uterine muscle contraction or uterine atony (a loss of tone in the uterine musculature), however, can also lead to an acute haemorrhage, as the uterine blood vessels are not sufficiently compressed. This is where the management of PPH comes in.
Uterine massage is a simple first line treatment as it helps the uterus to contract to reduce bleeding (Hofmeyr, et al. 2008). Although the evidence around the effectiveness of uterine massage is inconclusive, it is common practice after the delivery of the placenta.
The WHO recommendations (WHO, 2012) include intravenous oxytocin. Ergotamine may also be used (Weeks, 2015).
Abedi, P., Jahanfar, S., Namvar, F., & Lee, J. (2016). Breastfeeding or nipple stimulation for reducing postpartum haemorrhage in the third stage of labour. Cochrane Database of Systematic Reviews, (1). doi: 10.1002/14651858.CD010845.pub2. PMID: 26816300
Carroll, R. G. (2007). Elsevier's Integrated Physiology Chapter 16 - Life Span, Philadelphia, pp 197-208. PA: Mosby doi: 10.1016/B978-0-323-04318-2.50022-4
Hofmeyr, G. J., Abdel‐Aleem, H., & Abdel‐Aleem, M. A. (2008). Uterine massage for preventing postpartum haemorrhage. Cochrane Database of Systematic Reviews, (3). doi: 10.1002/14651858.CD006431.pub2 PMID: 18646154
Weeks, A. (2015). The prevention and treatment of postpartum haemorrhage: what do we know, and where do we go to next?. BJOG: An International Journal of Obstetrics & Gynaecology, 122(2), 202-210. doi: 10.1111/1471-0528.13098
WHO (2012). WHO recommendations for the prevention and treatment of postpartum haemorrhage. Geneva: World Health Organization.
Available in 5 languages via http://www.who.int/reproductivehealth/publications/maternal_perinatal_health/9789241548502/en