Fetal Development

Fetal Development – The First Nine Months

Pregnancy can seem strange and mysterious. If you’ve recently found out you’re pregnant, or you’re just curious about the development of an unborn child, you’ve reached the right place. Here we’ll take a look at the fetal development and growth of your pregnancy week by week, so you can learn about and enjoy the wonderful changes that occur throughout every pregnancy.

When Does Pregnancy Start?

Before embarking on the journey of a child’s development in the womb, we must first determine exactly when the pregnancy began. In modern medicine, there are two ways of dating a pregnancy: gestational age and fertilization age.

Gestational age refers to the time since the first day of the mother’s last menstrual period. In most visits to a prenatal doctor, he or she will refer to the age of your baby in terms of gestational age. In most cases, the gestational age begins two or more weeks before the fertilization of the egg by sperm.

Fertilization age refers to how long it has been since the actual event of conception: the fertilization of an egg by sperm. Because most women ovulate (release an egg into their womb from the ovary) in the middle of their menstrual cycle, fertilization age is typically about 14 days later than the gestational age. However, every woman’s cycle is different and thus fertilization age is slightly less accurate than gestational age.

For purposes of convenience, the dates in this development guide use gestational age as a milestone. To determine when these milestones occur in terms of fertilization age, simply subtract approximately two weeks from our week by week pregnancy fetal development guide.

The Stages of Pregnancy

Fetal Development
Two Weeks After Your Period (Conception Day)
The egg and sperm come together, most often in the fallopian tubes, to form a single cell known as a zygote. The zygote contains all of the genetic information present in an adult, including height, eye color, hair color, gender and more.

As the single-celled zygote divides and grows, it becomes what we call an embryo. As the embryo gets larger it travels down the fallopian tube and eventually reaches the uterus about three or four days after conception. At this point, the embryo is about the size of a grain of sand.

Three Weeks
By the sixth or seventh day of pregnancy the embryo implants itself into the lining of the uterus, triggering pregnancy responses in the mother. At this point, blood tests can show that the mother is pregnant.
Four Weeks: By one month after the mother’s last period, the embryo is now fully connected to the mother’s bloodstream via the uterus. At this point, urine tests can detect pregnancy hormones accurately.
Five Weeks
The embryo is developing rapidly and beginning to form internal organs. The heart is the first to begin functioning, and it starts to beat at about this point. Other organ systems including the brain, lungs, and stomach, are beginning to form as well.
Six Weeks
The embryo is now about the size of a large sweetpea. The brain, spinal cord, and central nervous system is almost fully formed, and the eyes, arms, and legs are visibly developing. The embryo’s heart beats about 120 times every minute – twice per second – at this stage of pregnancy.
Seven Weeks
The baby’s circulatory system is almost fully formed, and it has begun to create its own blood inside it’s body. Depending on whether the embryo is male or female, it is beginning to form either testicles or ovaries.
Eight Weeks
The embryo is now slightly smaller than a U.S. dime. Fingers, toes, elbows, and knees are forming on the limbs, and the embryo is able to move them in response to touch by reflex. The embryo’s lungs, tongue, and tooth buds are also forming at this pregnancy stage.
Nine Weeks
The development of the pregnancy keeps accelerating. By nine weeks, the embryo is about the same as a penny. Limbs, fingers, and toes continue to grow and are beginning to form hard bones inside. The eyes, ears, and nose are also forming rapidly, and the eyes even begin to take on color in the retina.
Ten Weeks
By this stage of pregnancy the embryo is rapidly expanding its nervous system, limbs, and internal and external organs. The baby can begin purposely moving its arms and legs.
Eleven Weeks
After about three months of pregnancy, we no longer refer to the growing baby as an embryo. Instead we use the Latin word fetus, which means “young one.” At this stage the fetus has all of its major organ systems in place and is beginning to look recognizably like a baby. It is around the same size as a lime. The fetus can perform many familiar motions including yawning, sucking, and stretching. The eyelids are developed and close to protect the eyes, and the kidneys begin to produce urine.

At this point, the baby’s internal organ systems are almost fully developed, and its external appearance is changing rapidly as it grows larger. Fetal development week by week is at this point mostly focused on growth.

Fourteen Weeks
The fetus is about three inches long, somewhere around the size of a peach. It is growing fingernails and toenails, and the fetus can usually find its own thumb and suck on it in the womb.
Sixteen Weeks
By four months, the fetus is about the size and weight of a large avocado. The heart is beating very fast at this stage of pregnancy: about 110 to 180 times per minute (three times every second!). Ultrasounds at this stage can often determine the gender of the fetus.
Eighteen Weeks
The baby’s internal development continues, with the skeleton growing and hardening. External development is also progressing well: the baby can blink and frown, and the fingers and toes have developed the prints they will have for life.
Twenty Weeks
You may be feeling the fetus move inside you regularly now, as it can easily coordinate its arm and leg movements to squirm and kick. It is about ten inches long and weighs nearly 11 ounces, about the size of a pomegranate. The baby is developing sleeping patterns, and some studies suggest it can also respond to pain signals. This pregnancy stage also marks the approximate “halfway point” for the pregnancy.
Twenty-two Weeks
The fetus is about 12 to 14 inches long from head to heel, and weighs almost a pound (16 ounces). Hair is beginning to develop on the head and skin.
Twenty-four Weeks
The fetus is about the size of a head of lettuce. The face is fully formed by this stage, and it has begun to practice breathing by inhaling and exhaling amniotic fluid. Most fetuses at this point can also hear voices and other sounds from outside the womb, and it may begin to recognize your voice, breathing, and heartbeat. They can even be “startled” by noises applied to the mother’s abdomen.
Twenty-six Weeks
Fetal development is beginning to reach the final stages as the mother enters her third trimester. The fetus weighs about two pounds and is coming up to 16 inches in length, about the size of a large eggplant. The eyes are almost fully-formed and can respond to light, and the teeth buds are permanently in place for baby teeth to start growing. The baby is also growing eyebrows and eyelashes.
Twenty-eight Weeks
With the support of intensive care, many babies born at this stage stand a good chance of survival. The brain is developing the ability to regulate breathing and body temperature, and the immune system is starting to support itself. The most important stages of fetal development are finished at this point – from here on, the baby mostly just grows larger.
Thirty-four Weeks
Fetal development is nearly finished, and the baby is preparing to be born. Many babies are extremely active in the womb by this point, and have developed reflexes including light, sound, and touch responses. The lungs are still in their final stages of development, so babies born at this stage may still need some assistance breathing. On average, babies at this development stage are nearly five pounds in weight.
Fourty Weeks
On average a baby at this stage is nearly 20 inches in length. Most babies naturally position themselves head-down in the uterus and start to migrate down toward the pelvis in preparation for birth.

The typical “due date” is forty weeks after conception, but only about four percent of babies are actually born on their due dates. It’s not uncommon for pregnancies to go on a week or two after the due date.

It’s important to remember that every pregnancy is different, and your week by week experience may be different from that of other women. However, one thing is certain – when you find out you’re pregnant, you should schedule an appointment with a prenatal care doctor or midwife who can advise you on the best ways to care for yourself and your developing child. A doctor or midwife can also answer all your questions and give you the best advice for the remainder of your pregnancy.

If you’re scared or unsure about your pregnancy, give Hope a call. We have years of experience in helping women through the confusion and difficulty associated with an unplanned or surprise pregnancy. We offer hope, support, and education – never judgment.

Have Questions? Please text (405) 531-9011 or schedule your appointment.