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.

Month 2: This month, the embryo’s development shifts into high gear. Its tongue, teeth and eyelids start to form. Its limbs grow longer and stronger, and its palate is nearly complete. Also in this time period, the embryo’s gastrointestinal tract separates from its urogenital organs and its heart begins beating—twice as fast as yours, in fact.   The essential organs all are under construction and major body parts are in place (legs, arms, head and torso). As the eighth week of pregnancy draws to a close, one more major development unfolds. The embryo gets a final name change, and it is now called a “fetus” until birth.

Month 3: This is the month of the heartbeat. Using a special tool called a Doppler monitor, doctors can detect the tiny thump-thumps of a 10-week-old fetus.   This is also a time of rapid growth inside the womb. By month’s end, the fetus will weight roughly 1 ounce, and it will double in length, uncurling from a tight C-position until it’s about 3 inches long. The tail will disappear and its eyelids, earlobes, limbs and digits will continue to form.   Other developmental milestones for this period include thumb-sucking, head-nodding and balling tiny fingers into fists. And though the fetus’s reproductive organs now are under construction, it’s still a bit early for the doctor’s gender declaration. That news (should you want it) usually arrives early in pregnancy’s second trimester.

Month 4: Get out the headphones and tune the radio dial to Mozart—this month, the fetus can hear its mother’s heartbeat, her voice and other outside noises. The fetus is also developing at warp speed; by now, all its major organs are complete.   In addition, its bones are growing stronger and its muscles longer. Its reflexes also are sharpening up—it can now swallow, kick and execute an occasional somersault with relative ease. And by month’s end, the baby-to-be will weigh around 6 ounces and stretch some 7 inches long. Sweat glands will dot its palms and soles, and its wrinkly skin will shine transparent pink.   At this stage, the baby-to-be may look like a complete package, but it still has a few more months of growing to do. By the end of month four, most doctors can perform an ultrasound test and identify the baby’s gender.

Month 5: Finally, the baby really starts kicking! “Quickening” is when a mother senses her potential punter in action for the first time, and this milestone moment usually happens during pregnancy’s fifth month.   Also new in this month: hair. The fetus now has tiny white eyelashes and two arching eyebrows. Fine woolly hairs, called lanugo, blanket its body and its bare head also sports a few sprouts.   In addition to lanugo, a white, waxy substance called vernix caseosa covers and protects the fetus’s body. And by month’s end, the fetus measures 8 to 10 inches long and tips the scales at 1 pound.

Month 6: Month 6 marks the end of the second trimester. At this point, the fetus’s essential organs—its kidneys, heart and lungs—are fully formed. The facial features are becoming more recognizable. It also can hiccup and react to loud “outside” noises by blinking, shifting or kicking. By month’s end, the fetus will measure about 12 or 13 inches long and weigh roughly 2 pounds. 

Month 7: The 7-month-old fetus can blink, and its eyes may even remain open for short period of time. Hands and feet are becoming even more active. Also in this phase: Taste buds form and protective fat tissue makes its debut. The fetus’s hearing is fully developed and, in boys, its testicles have moved to the groin. By month’s end, the baby-to-be will measure 14 to 16 inches long and weigh anywhere from 2 ½ to 3 ½ pounds.

Month 8: This month, the fetus’s brain develops rapidly, and all of its organs except the lungs are mature. An 8-month-old fetus stretches 16 to 18 inches long and weighs between 4 and 6 pounds. And as the baby-to-be grows larger, space in the womb becomes scarce. Expectant mothers should still count on catching a few elbows every day, but the elaborate somersault sequences should stop until delivery day. Other exciting changes during this period? The fetus’s fingernails now reach beyond its fingertips and its skin is starting to smooth.

Month 9: The finish line is in sight! In this final month of development, the fetus’s fat layers thicken to help keep it warm outside the womb, and the protective layers of vernix caseosa and lanugo largely disappear. By now, the fetus’s lungs are mature, its skin pink and smooth, and its toenails fully grown. The baby-to-be can also execute an array of reflexes, such as head turning, blinking and grasping. At this late stage, it stretches between 20 to 22 inches long, and weighs about 7½ pounds.   To prepare itself for delivery, the fetus changes position and drops down in its mother’s pelvis, usually with its head pointed toward her birth canal.

Sources: The National Women’s Health Information Center via the U.S. Department of Health and Human Service (www.4women.gov) and The Office of Public Health’s website, 4parents.gov (www.4parents.gov).