My father told me all the Weasleys have red hair, freckles, and more children than they can afford.
- Draco Malfoy
Ron's parents seem educated, so I wonder whether the wizarding world has devices of birth control:
Dragon Bladder condoms (Plastic is a muggle thing)
Contraceptive Potion
Sterilization Charm
etc.
Anything from the canon?
I can't think of any mention of such a thing in any canon sources, so I offer an alternative explanation of the Weasleys' family size.
Remember that "more children than they can afford" is only Malfoy's description. We do see the Weasleys worrying about money for new wands and broomsticks, and they always seem to be wearing hand-me-downs and reading second-hand books. But they don't seem to be seriously hurting for food. Given the Weasley parents' intense affection for all of their children, it seems far more likely that they just like having lots of children, and decided to prioritize that over new robes and a posh house. Malfoy sees that they can't afford new robes for all their children and concludes that they can't afford to have all their children in the first place, but that just means he needs to sort out his priorities, which we already knew.
Can you imagine Mr. and Mrs. Weasley looking at one of their children and thinking, "Ooh, yeah, we shouldn't have had that one"? Not a chance.
Compare the Malfoys, who are rich and proud of their blood, with the Weasleys, who are poor(ish) with red hair and lots of children, and are regarded as socially and morally inferior by the ruling elites. Does this remind anyone of ... the English looking down on the Irish? The Irish have been largely Roman Catholic for a long time (recent secularization notwithstanding), and Catholics don't use artificial birth control. (More here, with a bonus Spanish translation.) (Edit: I know some Catholics do use artificial birth control. I meant it in the sense that or "Catholics don't murder," or "Catholics don't shack up without getting married." The Church teaches that it's wrong, but some still do it. The 98% statistic you've probably heard is super misleading, but at least in the US, catechesis on this topic is frequently missing or unhelpful, so lots of Catholics don't understand why it's wrong, or what to do "instead," and how. If you're interested in more information on this topic, feel free to ping me in chat.)
I'm not claiming that the Weasleys are Catholic (it's a theory, though). But giving them this kind of Catholic-ish trait fits the rest of their image and role in the story.
From canon:
‘I’ve got something for you, Harry,’ said Hermione, neither looking at Ron nor giving any sign that she had heard him. ‘Oh, hang on – password. Abstinence.’
‘Precisely,’ said the Fat Lady in a feeble voice, and swung forwards to reveal the portrait hole.
‘What’s up with her?’ asked Harry.
‘Overindulged over Christmas, apparently,’ said Hermione, rolling her eyes as she led the way into the packed common room. ‘She and her friend Violet drank their way through all the wine in that picture of drunk monks down by the Charms corridor. Anyway ...’
Half-Blood Prince - Chapter Seventeen, A Sluggish Memory - Page 329 - Bloomsbury
Abstinence is a form of birth control; how legitimate one holds abstinence as a useful form of contraception will vary person to person. Or wizard to wizard, as it were. Now some may find abstinence as a precarious or useless form of birth control, it is a form of contraception that some Muggles use -- so why not witches and wizards? Anyhow, this is the only direct canon reference to birth control that comes to mind for me.
Funny question!
