Ten to 14 days after fertilization, the egg, now considered an embryo, is about the size of a poppy seed. At 21 to 22 days, the heart begins to beat.
The embryo is approximately 0.01 inches (0.36 millimeters) long, smaller than a grain of rice.
Second Month
* The brain is creating 2.5 million brain cells every minute it is in the womb; it will have more than 100 billion cells by the time the baby is born.
* The uterus expands from the size of a lemon to the size of a grapefruit. The uterus has the ability to ultimately expand to the size of a large watermelon.
* The embryo is a little more than half an inch (17 millimeters) long, or the size of a lima bean.
Third Month
* Almost all of the parts of the body have formed—eyes, ears, nose, fingers, arms and legs—and continue to grow.
* The embryo is now a fetus, weighing 1.5 ounces (42.5 grams) and measuring approximately 2.5–3 inches (6.4–7.6 centimeters).
* A mother-to-be will go for her first ultrasound scan at 10 to 14 weeks into the pregnancy
Fourth Month
* The womb is now the size of a small melon. The placenta, the organ that processes food and oxygen for the fetus, is fully functioning.
* Arms and legs are fully formed and moving. The mother may begin to feel the fetus' movements, sometimes called "quickening."
* The brain is developing neural pathways that allow the fetus to sense other parts of the body and elements in its environment. As the fetus begins to move, it also develops an awareness of the space around it. This awareness or proprioception helps humans navigate the constantly changing world around us. Twins and higher order births develop this sense with the help of their siblings when one kicks or moves and another fetus responds.
* The digestive system begins to function and the fetus learns to swallow by drinking some of the amniotic fluid. Some of the flavor from the mother's food will be transferred into the amniotic fluid and can influence the development of taste buds. Since multiples are sampling the same foods in the womb, they may have similar tastes in foods once they grow up.
Fifth Month
* A single fetus is halfway to birth, weighing almost one pound (0.45 kilograms) and measuring 7–9 inches (18–23 centimeters).
* The fifth month is an important time for a fetus to grow, but for multiples, the presence of two or more fetuses means room to grow is limited. To decrease the likelihood of premature birth, the growth rate for multiples gradually slows after five months in the womb.
* A male fetus' penis and scrotal sac are enlarged and look like they are growing out from the fetus' abdomen, and are visible on an ultrasound exam. The testes will not descend until the end of pregnancy.
Sixth Month
* The growth rate of multiples tends to slow.
* A female fetus' ovaries are developing all the egg cells she will have throughout her lifetime.
* The fetus' eyes open for the first time.
Seventh Month
* Hair is growing and beginning to develop the color it will have upon birth. Nails have formed on the fingers and toes.
* Triplets' growth rate tends to slow. Triplets are frequently born at 34 weeks.
Eighth Month
* Twins' growth rate tends to slow.
Ninth Month
* Many twins are born at 36 weeks.
* The umbilical cord, the baby's lifeline, has grown to almost 2 feet (0.6 meters) long, and the placenta weighs about 1.5 pounds (0.7 kilograms).
Fourth Month
