Month 1: Human development starts when a sperm fuses with an egg to create a zygote

Month 1: Human development starts when a sperm fuses with an egg to create a zygote. A zygote is a single-cell that contains two copies of chromosomes—one copy from each parent. In the week following fertilization, the zygote undergoes rapid cell division and becomes a mass of cells known as a blastocyst. After more cell division, the blastocyst splits in half.

One half of the blastocyst will become the placenta and the other half will become the fetus. But just 12 days after fertilization, the cells destined to become the fetus turn into something else first: the embryo.

By the month’s end, this embryo is about the size of a grain of rice, and it looks more tadpole than human, donning a tail in lieu of legs. But already, its surface features and major organs—the heart, lungs, liver and kidneys—are starting to take shape.

Attention expectant moms: Exposure to high heat during early pregnancy may be risky business. Researchers at Boston University found that if expectant moms climbed into a hot tub or sauna or suffered a high fever during their first trimester of pregnancy, their baby faced an increased risk of developing neural tube defects (NTDs). Lounging in a hot tub is riskier than exposure to a sauna or fever, according to the study, which was published in the medical journal JAMA. The researchers also found that cuddling up with an electric blanket during the first few months of pregnancy posed no risk for NTDs.