What are the stages of fetal development?
And when does pregnancy start?
What happens immediately after pregnancy?
What is the timeline of fetal development?
The answers are in pictures in this report.
When does pregnancy start?
The onset of pregnancy is on the first day of the last menstrual period, and this is called the "gestational age", or the "menstrual age".
It's about two weeks before you're due to actually get pregnant, according to the Cleveland Clinic website.
The date of the first day of your last period will be an important date. Your health care provider will ask you about this date and will use it to see how far along you are with your pregnancy.
How does pregnancy happen?
Every month a woman's body goes through a reproductive cycle that can end in one of two ways;
You will either have a menstrual period or become pregnant, and this cycle occurs continuously during your childbearing years;
From puberty in the teenage years to menopause in about 50 years.
In the cycle that ends with pregnancy, there are several steps:
A group of eggs prepares to leave the ovary through ovulation.
The eggs develop in small fluid-filled sacs called sacs.
Follicles are small containers for each immature egg.
Of this group of eggs only one will mature and persist, after which this vesicle (or follicle) suppresses all other sacs in the group, and the other sacs stop growing at this stage.
The mature follicle opens and releases the egg from the ovary;
This is ovulation.
Ovulation generally occurs two weeks before the start of your next period.
It is generally in the middle of your cycle.
After ovulation, the open (ruptured) follicle develops into a structure called the corpus luteum.
It secretes the hormones progesterone and estrogen.
And progesterone helps prepare the lining of the uterus.
This lining is where the fertilized egg settles to develop.
If you don't get pregnant during your period, this is the lining that sheds during your period.
On average, fertilization occurs about two weeks after your last period.
When the sperm penetrates the egg, changes occur in the protein coat of the egg to prevent other sperm from entering.
At the moment of fertilization, your baby's genetic makeup, including its gender, is complete.
The sex of your baby depends on the quality of the sperm that fertilizes the egg at the moment of conception.
Women have an XX genotype and men have an XY.
Women mark each egg with an (X).
Each sperm can be either an "X" or a "Y".
If the fertilized egg and sperm are a mixture of X and Y then it is a boy, and if there are two Xs, then it is a girl.
Fetal development during the first trimester of pregnancy (Getty Images)
What happens immediately after pregnancy?
Within 24 hours after fertilization, the egg begins dividing rapidly into several cells.
The fertilized egg remains in the fallopian tube for 3 days after conception.
The fertilized egg (now called a blastocyst) continues to divide and slowly passes through the fallopian tube to the uterus.
Once there, its next job is to attach to the lining of the uterus;
This is called implantation.
Before implantation, the blastocyst ruptures from its protective shell.
When the blastocyst comes into contact with the endometrium, some women notice slight bleeding during the day or two of implantation.
This is normal and you should not worry about it.
At this point, the lining of the uterus becomes thicker and the cervix (the opening between the uterus and the birth canal) is closed with a plug of mucus.
Within 3 weeks, the blastocysts eventually form a small ball or embryo.
By this time the first neurons have been formed.
How early can I find out that I am pregnant?
From the moment of conception, the hormone human chorionic gonadotropin will be present in your blood.
This hormone is produced by the cells that make up the placenta (a developing fetus's food source).
It is also the hormone that is detected in a pregnancy test.
Although this hormone is present from the start, it takes time to build it in your body, and it usually takes 3 to 4 weeks from the first day of your last menstrual period for the human chorionic gonadotropin to increase enough to be detected by pregnancy tests.
What is the timeline of fetal development?
The fetus will change a lot throughout a typical pregnancy.
This time is divided into 3 phases, called trimesters.
You will notice clear changes in the fetus and in yourself during every 3 months.
Traditionally, we think of pregnancy as a 9-month process.
However, this is not always the case;
A full-term pregnancy is 40 weeks, or 280 days.
Depending on the months you are pregnant (some shorter, some longer) and the week you get pregnant, you could be 9 months or 10 months pregnant.
This is completely normal and healthy.
Fetal development in the first trimester of pregnancy
The first trimester of pregnancy extends from the beginning of pregnancy to 12 weeks;
During which the fertilized egg will change from a small group of cells into an embryo that begins forming the features of the child.
Fetal development in the first month
As the fertilized egg grows, a water-tight sac forms around it, gradually filling with fluid.
This is called the amniotic sac, and it helps cushion and protect the fetus.
During this time, the placenta is also developing;
The placenta is a flat, circular organ that transports nutrients from the mother to the fetus and wastes from the fetus.
The placenta is a source of nourishment for the fetus throughout pregnancy.
In these first few weeks, the face begins with large dark circles of the eyes, the mouth, lower jaw and throat develop, blood cells are formed, blood circulation will begin, and the small "heart" tube will beat 65 times per minute by the end of the fourth week.
By the end of the first month, the fetus is about a quarter of an inch long;
Smaller than a grain of rice.
(An inch equals 2.54 centimeters).
Fetal growth in the second month
Facial features continue to develop, and each ear begins as a small fold of skin on the side of the head, and small buds form that eventually grow into the arms and legs, as are the fingers, toes, and eyes.
The neural tube (the brain, spinal cord, and other nerve tissue of the central nervous system) is now well formed, the digestive system and sensory organs begin to develop as well, and the bone begins to replace cartilage.
The head is large in relation to the rest of the body at this point;
In about 6 weeks, a heartbeat can usually be detected.
By the end of the second month, the fetus is about an inch long and weighs about a third of an ounce (less than 10 grams), according to WebMD.
Fetal growth in the third month
The arms, hands, fingers, feet and toes are fully shaped.
At this stage, the fetus is beginning to explore a bit by doing things like opening and closing its fists and mouth.
Fingernails and toenails begin to develop and the outer ears are formed.
The beginnings of the teeth form under the gums, and the genitals also develop, but it is still difficult to distinguish the sexes on ultrasound.
By the end of the third month, the fetus is fully formed;
All organs and limbs are present and will continue to develop, the circulatory system and urinary system also work and the liver produces bile.
At the end of the third month, the fetus is about 4 inches long and weighs about 1 ounce.
(An ounce equals about 28 grams).
Fetal development in the fourth month
The fetus's heartbeat may now be heard with an instrument called a Doppler.
Fingers and toes are well defined.
Eyelids, eyebrows, eyelashes, nails and hair are formed.
Teeth and bones become denser.
The fetus can suck its thumb, yawn, stretch and make faces.
The nervous system started working.
Your genitals and reproductive organs are now fully developed, and your doctor can see on an ultrasound if the fetus will be identified as male or female at birth.
By the end of the fourth month, the fetus is about 6 inches long and weighs about 4 ounces.
Fetal development in the fifth month
At this stage, you may feel that the fetus is moving, developing and exercising muscles.
Hair begins to grow on the head, and the shoulders and back are covered with fine hairs called lanugo.
This hair protects the fetus and usually falls out at the end of the first week of your baby's life.
The skin is covered with a white layer called the lid.
It is believed that this "cheesy" substance protects the fetus's skin from prolonged exposure to amniotic fluid, and this layer falls off just before birth.
By the end of the fifth month, the fetus is about 10 inches long and weighs 1/2 to 1 pound.
(A pound equals about 450 grams).
Fetal development in the sixth month
If you could look inside the womb now, you would notice that the fetus's skin is reddish, wrinkled, and the veins are visible through the clear skin.
Fingerprints and footprints appear.
At this point the eyelids begin to separate and the eyes open.
The fetus responds to sounds by moving or increasing the pulse, and you may notice jerky movements if the fetus hiccups.
By the end of the sixth month, the fetus is about 12 inches long and weighs about two pounds.
Fetal development in the seventh month
The fetus continues to mature.
At this point hearing is fully developed, the fetus frequently changes position, responds to stimuli, including sound, pain and light, and the amniotic fluid begins to contract.
At the end of the seventh month, the fetus is about 14 inches long and weighs 2 to 4 pounds.
Fetal development in the third trimester of pregnancy
The third trimester begins at 28 weeks of pregnancy and continues until delivery, around week 40, according to the American Pregnancy Association.
Fetal development in the eighth month
The fetus continues to mature, and you may notice more kicking, the brain is developing rapidly at this time, the fetus can see and hear, and most of its internal systems are well developed, but the lungs may still be immature.
The fetus is about 18 inches long and weighs 5 pounds.
Fetal development in the ninth month
During this stage the fetus continues to grow and mature, and the lungs are approaching full development at this stage.
The fetus has coordinated reflexes. It can blink, close its eyes, turn its head, grasp firmly, and respond to sounds, light and touch.
The fetus is about 17 to 19 inches long and weighs 5 to 6 pounds.
Fetal growth in the tenth month
In this last month, you can go into labor at any time, and you may notice less movement because space is tight.
At this stage, the fetus's position may change in preparation for birth.
Ideally, it's heading down the womb, and you may feel very uncomfortable in this last period of time as the fetus descends into your pelvis and prepares to be born.
Your baby is ready to meet the world at this stage, and is about 18 to 20 inches long, and weighs about 7 pounds.
How ultrasound used to track the stages of fetal development?
Fetal ultrasound is an imaging technique that uses sound waves to produce images of the fetus inside the womb.
A fetal ultrasound can help your health care provider evaluate your baby's growth and development and monitor your pregnancy.
In some cases, a fetal ultrasound is used to evaluate potential problems or help confirm a diagnosis, according to the Mayo Clinic.
"Usually the first ultrasound of the fetus is done during the first trimester to confirm pregnancy and estimate how far into your pregnancy," says the Mayo Clinic. If your pregnancy continues without complications, the next ultrasound is usually done during the second trimester, when Anatomical details are visible. If a problem is suspected, follow-up ultrasound or additional imaging tests, such as an MRI, may be recommended."
And the Mayo Clinic adds that, "Ultrasound examination is done during the first trimester of pregnancy to assess the presence, size and location of the pregnancy, determine the number of fetuses, and estimate the duration of your pregnancy (gestational age). In the second or third trimester, a regular ultrasound is done to evaluate several characteristics of pregnancy. Including the anatomy of the fetus. This test is usually done between weeks 18 and 20 of pregnancy. However, the timing of the ultrasound may be changed for reasons such as obesity, which may limit the ability to view the fetus."
"During the second and third trimesters of pregnancy, a limited ultrasound assessment may be required when a specific question is needed to answer. Examples include assessment of fetal growth and estimation of amniotic fluid volume," the Mayo Clinic explains.
The health care provider may use fetal ultrasound to confirm pregnancy and location, determine gestational age, confirm the number of fetuses, assess fetal growth, examine the placenta and amniotic fluid levels, identify birth defects, investigate complications, and perform prenatal tests. The other, determining the position of the fetus before birth.