Seeing the growth and development of a baby within a woman is an incredible experience. From the union of an egg and a sperm to the first, second, third, etc. division of the cells we can see a beautiful symphony of God’s creation of a human. How in the world do the cells know if they should develop into bones or muscles or cute little toes?
Fertilization
Fertilization: the joining of the egg and the sperm to form a unique human being. The zygote has 46 chromosomes – 23 from the mother and 23 from the father. These chromosomes will help determine the baby’s sex, traits such as eye and hair color, and, to some extent, personality and intelligence.
Soon after fertilization, the zygote travels down the fallopian tube toward the uterus. At the same time, it will begin dividing rapidly to form a cluster of cells resembling a tiny raspberry. The inner group of cells will become the embryo. The outer group of cells will become the membranes that nourish and protect it.
Week 4
When the zygote — now known as a blastocyst — reaches the uterus, it will burrow into the uterine wall for nourishment. The placenta, which will nourish the baby throughout the pregnancy, also begins to form. Hormones produced by the embryo have stopped the mother’s menstrual cycle.
* Technically, the age of this embryo is only 2 weeks since conception. Obstetricians count “weeks of pregnancy” from the first day of a woman’s last menstrual cycle (not from the date of actual fertilization) because there is often no way to determine exactly when conception occurred. For the purposes of these pages, we will count weeks of pregnancy as obstetricians do; from the first day of the last menstrual cycle. For the perspective of the age of the embryo since conception, subtract 2 weeks from the ages noted here.
Week 5
The fifth week of pregnancy, or the third week after conception, marks the beginning of the embryonic period. This is when the baby’s brain, spinal cord, heart and other organs begin to form.
Week 6
Growth is rapid this week. Just four weeks after conception, the neural tube along the baby’s back is closing and the baby’s heart is pumping blood.
Basic facial features will begin to appear, including passageways that will make up the inner ear and arches that will contribute to the jaw. The baby’s body begins to take on a c-shaped curvature. Small buds will soon become arms and legs.
By the end of this week, the baby may be 1/6 to 1/4 inch (4 to 6 millimeters) long.
Week 7
Seven weeks into the pregnancy, (or five weeks after conception,) the baby’s brain and face are rapidly developing. Tiny nostrils become visible, and the eye lenses begin to form. The arm buds that sprouted last week now take on the shape of paddles.
The baby has developed its own blood type, unique from the mother’s.
Week 8
Eight weeks into the pregnancy, or six weeks after conception, the baby’s arms and legs are growing longer, and fingers have begun to form.
The baby may begin to move this week, but the mother won’t be able to feel it yet. The upper lip and nose have formed. The trunk of the baby’s body is beginning to straighten.
At the end of week 8, the embryonic period ends and the fetal stage begins.
Second Trimester
Weeks 13-16
The baby can make sucking motions with his mouth now. The baby’s nervous system allows him to feel pain. (New England Journal of Medicine)
The baby can make active movements including kicking and even somersaults.
Muscle tissue and bone continue to form, creating a more complete skeleton. The baby’s sex becomes apparent during this time.
The scalp hair pattern has formed.
More developed facial muscles may lead to various facial expressions such as squinting or frowning.
Weeks 17-18
Fat stores begin to develop under the baby’s skin. The fat will provide energy and help keep the baby warm after birth.
The baby’s ears begin to stand out on the sides of his or her head. As the nerve endings from the baby’s brain “hook up” to the ears, the baby may hear the mother’s heart beating, her stomach rumbling or blood moving through the umbilical cord. He or she may even be startled by loud noises.
Week 20
The 20th week is halfway into the pregnancy. ( 18 weeks after conception) The baby’s delicate skin is protected with a greasy, cheese-like coating called vernix caseosa.
Fingernails and toenails appear.
The mother may be able to feel your baby’s movements at this point. This is known as quickening.
The baby’s sex organs are visible on ultrasound.
By now the baby may be about 6 1/3 inches (160 millimeters) long from crown to rump and weigh more than 11 ounces (320 grams).
Weeks 21- 24
The baby is becoming more active and can swallow. He begins to have rapid eye movements. The tongue will soon develop taste buds. Fingerprints and footprints are forming.
For boys, the testes are beginning to descend from the abdomen. For girls, the uterus and ovaries are in place – complete with a lifetime supply of eggs.
The baby is regularly sleeping and waking. Real hair is beginning to grow on his or her head.
With intensive medical care, some babies born this week may be able to survive.
Weeks 25-27
The baby’s hands are fully developed. Exploring the structures inside the mother’s uterus may become baby’s prime entertainment.
This week marks the end of the second trimester. At 27 weeks, or 25 weeks after conception, the baby’s lungs, liver and immune system are continuing to mature – and he or she has been growing like a weed.
The baby’s crown-to-rump length may have tripled since the 12-week mark.
Third Trimester
Week 28 – 30
The baby’s eyelids are partially open and eyelashes have formed. The baby is gaining weight, which is smoothing out many of the wrinkles in his or her skin.
The bones are fully developed, but they’re still soft and pliable.
The baby may have a good head of hair by this week.
Weeks 31-34
Thirty-one weeks into the pregnancy, or 29 weeks after conception, the baby’s sexual development continues. If the baby is a boy, his testicles are moving through the groin on their way into the scrotum. If your baby is a girl, her clitoris is now relatively prominent.
Although the baby’s lungs aren’t fully formed, he or she practices breathing. Your baby’s body begins absorbing vital minerals, such as iron and calcium.
The layer of soft, downy hair that has covered your baby’s skin for the past few months – known as lanugo – starts to fall off this week.
The baby’s kicks and jabs may be forceful.
Thirty-three weeks into pregnancy, or 31 weeks after conception, the baby’s pupils can constrict, dilate and detect light entering his or her eyes.
Birth
