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Friday 11 October 2019

The Placenta (Things We Don’t Know about Pregnancy Series #4)

The placenta is a complex and poorly understood organ. Found in placental mammals, it forms during pregnancy, starts to break up in the last few weeks, and is normally expelled within 30 minutes of the birth. Looking at it, it’s easy to feel intimidated by this mysterious, massive, living thing that is birthed and dies when your baby is birthed.

Placenta via Wikipedia Commons.


What is the placenta anyway?

The placenta is a two-sided disc. On the one side, the maternal placenta (stuck to the womb) develops from the mother’s tissues 7-12 days after conception; on the other, the foetal placenta forms 17-22 days after conception from the blastocyst after it burrows into and connects up with the mother’s blood supply. Scientists are still studying how the placenta forms.

A burrowing foetus and the placenta may explain why women have periods. Some scientists reckon that spontaneous decidualisation (the body preparing for pregnancy monthly “just in case” rather than in response to an embryo implanting) and shedding to evacuate the womb may have evolved to protect the mother from an aggressive burrowing foetus. Once formed, the placenta acts as a barrier between mother and baby: protecting her from its demands and it from her immune system. It passes oxygen, nutrients, waste products, and partitions the two blood supplies. It may be involved in infant thermoregulation, and makes hormones from 18-20 weeks, taking over from the corpeus luteum: a bunch of ovarian cells produced when an egg is released and that later dissolve. There may be other placental functions: scientists are still exploring.

At the end of its life cycle, the placenta starts to break up. For placenta-sharing twins, this can sufficiently limit their oxygen and nutrient supply that doctors will induce the mother early. We don’t know why it breaks up early, but it might help detach it from the womb wall so that it can be expelled in the “third stage of labour”. In conditions such as placenta accrete and placenta abruption, the placenta won’t detach or falls off far too soon.

During the “third stage of labour”, women are often given an oxytocin injection their thigh to stimulate contractions and help expel the placenta. For some reason, this reduces postpartum bleeding, but scientists think there may be conflating adverse effects, and more research is needed[1].

Morning sickness

Morning sickness may be linked to placental growth. When the placenta is most vulnerable to chemical disruption, sickness peaks. Doctors think morning sickness may be a healthy indictor of good placental development. We don’t know how to measure placenta health directly: later in pregnancy, it is judged by foetus health.

Strangely, studies have shown that eating less early in pregnancy leads to a bigger placenta[2]. This suggests a link between nausea and the hormones stimulating placental growth. It might also ensure the maternal side of the placenta doesn’t grow too fast, and the two distinct sides form. For other theories on morning sickness, see Things We Don’t Know about Pregnancy Series #1: Morning Sickness.

What can and can’t cross the placenta?

Some things cross the placenta, like oxygen. Other things, like bacteria, don’t. Or do they?

Bacteria by Geralt via Pixabay.

The placenta has long been thought sterile, but recently, some researchers have claimed to find healthy and diverse microbes – this remains controversial[3][4][5][6].

We still don’t know how the placenta controls what can and cannot pass. It’s possible viruses can get through where bacteria can’t because they’re smaller. Yet the placenta collects and stores some chemicals, including medications from the mother’s bloodstream. If doctors could understand what triggers the storing mechanism, they could design medications that could treat the mother and not affect the baby.

Modelling the placenta is hard because placental cells do not spontaneously grow into a placenta, and trophoblast starter cells do not divide. However, scientists have managed to get some growth using a microgravity bioreactor system to model shear stress and rotational forces that they think mimic the cells’ true environment.

Towards the end of your pregnancy, the placenta passes antibodies from the mother to the baby that weren’t allowed to transfer earlier, conveying passive immunity to the baby for about 3 months after birth. But only some kinds of antibodies can pass – ones acquired a while ago. If you get a disease and fight it off whilst pregnant, the new immune cells won’t be passed on, although the disease could be!

One Nature study suggested that schizophrenia could develops during pregnancy when “schizophrenic genes” are turned on in the placenta[7]. These findings are the first to link early life complications, genetic risk and mental health.

Immune privilege

Mostly when something foreign enters a woman’s body, her immune system attacks it. Scientists are still trying to uncover why she doesn’t also kill off a foetus. It’s possible many miscarriages are precisely this: the woman’s body rejecting the new growth. Understanding the process could reduce miscarriage frequency (~10% of pregnancies). The placenta is believed to be instrumental in conveying “immune privilege” to the foetus. Amongst its many activities, the placenta secretes neurokinin B – the chemical used by parasitic nematodes to avoid detection by the immune system of their host!

However, it’s not the only time when genetically different cells have been found living inside our bodies. There are microbes, and there is microchimerism. We all exhibit microchimerism. Because some cells cross the placenta, mothers keep some of their babies’ cells, and their offspring carry some of their mothers. Cells may also be exchanged between unborn twins, and younger siblings may carry some of their older brothers and sisters. What microchimeric cells do and how benign they are remains unsolved, but they have been implicated in brain health: fewer are found in the brains of women who get Alzheimer's[8][9].

Cord blood

After birth, the cord to the placenta is cut. However, some research suggests it would be better to wait, allowing the baby absorb nutrients from placental blood that could help it adapt and decrease chances of anaemia[10]. Scientists are interested in what special qualities placental blood may have. Others have suggested delaying cord cutting may be linked to increased risk of jaundice[11]. Cord blood can also be stored and used to treat rare blood diseases and leukaemia.

Eating your placenta

Some people make placental pills. Image via Pixabay.
I wasn’t planning on eating my placenta: but some people do. There are no proven benefits to it, but some believe it will improve energy, milk, mood and healing. However, cooking, dehydrating and processing the placenta destroys most of the proteins and the hormones, so these claims remain unlikely. In addition, there are risks: the placenta’s job is to act as a filter and hold back dangerous chemicals and bacteria, which means some of these may remain in its tissues. The conditions during birth do not conform to food hygiene standards, and contamination may occur. Ingesting something harmful also carries risk for the baby if the mother is breastfeeding.

There are many unknowns when it comes to pregnancy, and over the next few months, I’ll be exploring more of them with you. Look out for my next blog post, which will be about nesting.

Read our full article on the things we don't know about pregnancy, which will be updated as we add posts across the coming months.

why don't all references have links?

[1] Begley, Cecily M.; Gyte, Gillian M. L.; Devane, Declan; McGuire, William; Weeks, Andrew (2015-03-02). Active versus expectant management for women in the third stage of labour. The Cochrane Database of Systematic Reviews (3): CD007412. doi:10.1002/14651858.CD007412.pub4. ISSN 1469-493X. PMC 4026059. PMID 25730178.
[2] Huxley, Rachel R. Nausea and vomiting in early pregnancy: its role in placental development. Obstetrics & Gynecology 95.5 (2000): 779-782.
[3] Perez-Muñoz, Maria Elisa; Arrieta, Marie-Claire; Ramer-Tait, Amanda E.; Walter, Jens (2017). A critical assessment of the sterile womb and in utero colonization hypotheses: implications for research on the pioneer infant microbiome. Microbiome. 5 (1): 48. doi:10.1186/s40168-017-0268-4. ISSN 2049-2618. PMC 5410102. PMID 28454555.
[4] Mor, Gil; Kwon, Ja-Young (2015). Trophoblast-microbiome interaction: a new paradigm on immune regulation. American Journal of Obstetrics and Gynecology. 213 (4): S131–S137. doi:10.1016/j.ajog.2015.06.039. ISSN 0002-9378. PMID 26428492.
[5] Prince, Amanda L.; Antony, Kathleen M.; Chu, Derrick M.; Aagaard, Kjersti M. (2014). The microbiome, parturition, and timing of birth: more questions than answers. Journal of ReproductiveImmunology. 104–105: 12–19. doi:10.1016/j.jri.2014.03.006. ISSN 0165-0378. PMC 4157949. PMID 24793619.
[6] Hornef, M; Penders, J (2017). Does a prenatal bacterial microbiota exist?. Mucosal Immunology. 10 (3): 598–601. doi:10.1038/mi.2016.141. PMID 28120852.
[7] Ursini, Gianluca, et al. Convergence of placenta biology and genetic risk for schizophrenia. Nature medicine 24.6 (2018): 792.
[8] Chan, William FN, et al. Male microchimerism in the human female brain. PLoS One 7.9 (2012): e45592.
[9] Gammill, Hilary S., et al. Effect of parity on fetal and maternal microchimerism: interaction of grafts within a host?. Blood 116.15 (2010): 2706-2712.
[10] Mercer JS, Vohr BR, Erickson-Owens DA, Padbury JF, Oh W (2010). Seven-month developmental outcomes of very low birth weight infants enrolled in a randomized controlled trial of delayed versus immediate cord clamping. Journal of Perinatology. 30 (1): 11–6. doi:10.1038/jp.2009.170. PMC 2799542. PMID 19847185.
[11] Emily Ostler, Expecting Better: Why the Conventional Pregnancy Wisdom is Wrong - and What You Really Need to Know (2013) Penguin Press.

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