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The physical connection between mother and fetus is provided by the placenta, an organ, built of cells from both the mother and fetus, which serves as a conduit for the exchange of nutrients, gasses, and wastes. Cells may migrate through the placenta between the mother and the fetus, taking up residence in many organs of the body including the lung, thyroid, muscle, liver, heart, kidney and skin. These may have a broad range of impacts, from tissue repair and cancer prevention to sparking immune disorders.

Meiosis, the form of cell division unique to egg and sperm production, sets the stage for sex determination by creating sperm that carry either an X or a Y sex chromosome. But what is it about the X or Y that determines sex? Before a meiotic cell divides, its two sets of chromosomes come together and cross over, or swap, segments. The first animation shows normal crossing over, where the X and Y chromosomes exchange pieces only at their tips. The second animation shows a rare mistake in which the Y chromosome transfers a gene called SRY to the X chromosome, resulting in sex-reversed babies. Studies of sex-reversed individuals led researchers to identify the master switch for sex determination, the SRY gene, which tells a fetus to become a boy.

Meiosis, the form of cell division unique to egg and sperm production, sets the stage for sex determination by creating sperm that carry either an X or a Y sex chromosome. But what is it about the X or Y that determines sex? Before a meiotic cell divides, its two sets of chromosomes come together and cross over, or swap, segments. The first animation shows normal crossing over, where the X and Y chromosomes exchange pieces only at their tips. The second animation shows a rare mistake in which the Y chromosome transfers a gene called SRY to the X chromosome, resulting in sex-reversed babies. Studies of sex-reversed individuals led researchers to identify the master switch for sex determination, the SRY gene, which tells a fetus to become a boy.

In both cases, researchers found that ZIKV-177 reduced levels of Zika virus in pregnant mice and their fetuses, compared with pregnant mice that were not given the antibody.

Unborn babies can sow the seeds for rheumatoid arthritis in their mothers - and the dads might be to blame. Rheumatoid arthritis is an autoimmune disease, meaning the body's immune system turns on itself. In this case, it causes painful, swollen joints. Women are three times as likely to develop the condition as men, and seem to be especially vulnerable soon after pregnancy. A mother exchanges cells with the fetus while it is in the womb. "For most women, shortly after you give birth, the fetal cells clear up," says Giovanna Cruz, an epidemiologist at the University of California at Berkeley. "But in a subset of women they actually persist for decades." In these women, the fetal cells are effectively incorporated into their bodies, a process known as microchimerism.

The Visible Embryo is a visual guide through fetal development from fertilization through pregnancy to birth. As the most profound physiologic changes occur in the "first trimester" of pregnancy, these Carnegie stages are given prominence on the birth spiral.