The placenta is usually functioning to deliver nutrients and oxygen to, and remove waste from, the embryonic blood by the week of pregnancy. the placenta is usually functioning to deliver nutrients and oxygen to, and remove waste from, the embryonic blood by the week of pregnancy.

The answer is the third week.  The egg is now a "blastocyst" burrowing into the blood-rich uterine lining. Some signs of pregnancy at 3 weeks and the few weeks following are Implantation bleeding. If your little soon-to-be-embryo has previously made it to his or her new home, you may see a bit of spotting as the fertilized egg burrows into the wall of your uterus.

individual oxygen atoms are formed during photosynthesis.-c.