What Actually Starts Labour? (Scientists NOW Finally Know)

Join my Skool to connect with other mums who value this kind of content! https://www.skool.com/parenting-with-... We know what happens during labour in huge detail, but for decades nobody could say what actually starts it. It's especially puzzling in humans, because progesterone (the hormone that keeps the uterus relaxed, like a brake pedal) stays high right through labour. In this video I look at a new study from Stephen Lye's lab at Mount Sinai Hospital in Toronto. It suggests that immune cells called macrophages move into the womb muscle near the end of pregnancy. They build tiny bridges called tunnelling nanotubes and hand over their own mitochondria, the cell's "power stations". This fuels the muscle for contractions. It also switches on an enzyme that breaks down progesterone inside the muscle, which takes the brake off. I also cover why this matters for preterm birth, and the honest caveats: it's still a preprint, the human work was done in cells in a dish, and the evidence from a living body comes from mice. 📚 Featured study: Nadeem L, Sharma A, Shankar S, Aguiar-Cabeza E, Pelletier L, Juriscova A, Shynlova O, Lye SJ. Macrophage to Myocyte Mitochondrial Transfer in the Myometrium: A Novel Mechanism for the Initiation of Labor. bioRxiv preprint, 2026 (not yet peer-reviewed). https://doi.org/10.64898/2026.08.17.7... 🔬 Want evidence-based parenting research in your inbox? Join the newsletter at https://joannalbyrne.com 💬 Did your labour start on its own, or did it need a helping hand? Tell me in the comments. And if your baby came early, I'd love to hear your story too. 👍 If you found this fascinating, please like and subscribe. It genuinely helps the channel reach more parents. Studies referenced: • Nadeem L et al. (2026). Macrophage to Myocyte Mitochondrial Transfer in the Myometrium. bioRxiv. https://doi.org/10.64898/2026.08.17.7... • Nadeem L et al. (2016). Molecular evidence of functional progesterone withdrawal in human myometrium. Nature Communications 7:11565. https://doi.org/10.1038/ncomms11565 • Nadeem L et al. (2021). Pro-inflammatory signals induce 20α-HSD expression in myometrial cells. Journal of Cellular and Molecular Medicine 25:6773–85. • WHO/UNICEF (2023). Born Too Soon: Decade of Action on Preterm Birth. https://www.who.int/publications/i/it... Follow my instagram @joannalbyrne Or Facebook @Joannalbyrne For any professional enquiries, please contact me via missjlstewart97@gmail.com