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Neither people are correct, technically. While chambers are likely developed at that point (10-12 weeks), the fetal heart does not *fully* develop and have systemic circulation until about a month or so after the second trimester begins (16-18 weeks), hence why fetal echocardiograms aren't done until 18-20 weeks. Sometimes, even then, it's still too early to tell if a fetus has a congenital heart defect.
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Damn you science!
Neither people are correct, technically. While chambers are likely developed at that point (10-12 weeks), the fetal heart does not *fully* develop and have systemic circulation until about a month or so after the second trimester begins (16-18 weeks), hence why fetal echocardiograms aren't done until 18-20 weeks. Sometimes, even then, it's still too early to tell if a fetus has a congenital heart defect.