I’m much older than you, have 2 children and will have a third next year (you are not old at all although I know it feels like it when you’re unsure of your own fertility with a first baby).
I am applying for GEM for 2021 or 2022 entry so cannot yet comment on the specific demands of med school but I would strongly recommend not underestimating the impact your first baby has on every aspect of your life. The massive adjustment to parenthood, the endless worry, the constant change as the baby grows and reaches sometimes difficult developmental milestones, the utter exhaustion and demands of trying to raise your little people ‘right’ are extremely all-encompassing.
I’m not saying don’t have baby now by any means...you must make the decision that is right for you...but give it extremely careful thought. You won’t be (or feel like) the same person you are now once you become a parent and your decision making and thought processes will be different. I wouldn’t assume you’ll feel comfortable going back to full time study and demanding placements when your baby is 5-6 months old, still possibly breastfeeding, not sleeping through the night and really still very small. Its so hard to imagine or plan for but it really does change everything. Make sure you think about what you’ll do if things change and you want to take more time off, would your uni allow that? The last thing you need as a new parent is any extra pressure, worry, guilt and demands on your time.