The baby’s sex is determined at conception when the sperm fertilizes the egg with either a X (female) or Y (male) chromosome. The sex is one of the 46 chromosomes that makes up the genetic code of an embryo. The baby’s sex is fixed at conception and doesn’t change as the pregnancy progresses.Visually, you can see the baby’s sex using ultrasound as early as 10 – 12 weeks. DNA testing can detect it earlier, at 6 – 9 weeks. And even earlier on day 5 of embryo testing.