Fetal Development Week By Week Descriptive Essay

The start of pregnancy is actually the first day of your last menstrual period. This is called the 'menstrual age' and is about two weeks ahead of when conception actually occurs.

Here's a primer on conception

Each month a group of eggs (called oocytes) is recruited from the ovary for ovulation (release of the egg). The eggs develop in small fluid-filled cysts called follicles. Normally, one follicle in the group is selected to complete maturation. This dominant follicle suppresses all the other follicles in the group, which stop growing and degenerate.

The mature follicle opens and releases the egg from the ovary (ovulation). Ovulation generally occurs about two weeks before a woman's next menstrual period begins.

After ovulation, the ruptured follicle develops into a structure called the corpus luteum, which secretes progesterone and estrogen. The progesterone helps prepare the endometrium (lining of the uterus) for the embryo to implant.

On average, fertilization occurs about two weeks after your last menstrual period. When the sperm penetrates the egg, changes occur in the protein coating around it to prevent other sperm from entering. At the moment of fertilization, your baby's genetic make-up is complete, including its sex.

If a Y sperm fertilizes the egg, your baby will be a boy; if an X sperm fertilizes the egg, your baby will be a girl.

Human chorionic gonadotrophin (hCG) is a hormone present in your blood from the time of conception. It is produced by cells that form the placenta and is the hormone detected in a pregnancy test. However, it usually takes three to four weeks from the first day of your last period for the hCG to increase enough to be detected by pregnancy tests.

Within 24 hours after fertilization, the egg begins dividing rapidly into many cells. It remains in the fallopian tube for about three days. The fertilized egg (called a blastocyte) continues to divide as it passes slowly through the fallopian tube to the uterus where its next job is to attach to the endometrium (a process called implantation). Before this happens, the blastocyte breaks out of its protective covering. When the blastocyte establishes contact with the endometrium, an exchange of hormones helps the blastocyte attach. Some women notice spotting (or slight bleeding) for one or two days around the time of implantation. The endometrium becomes thicker and the cervix is sealed by a plug of mucus.

Within three weeks, the blastocyte cells ultimately form a little ball, or an embryo, and the baby's first nerve cells have already formed. Your developing baby is called an embryo from the moment of conception to the eighth week of pregnancy. After the eighth week and until the moment of birth, your developing baby is called a fetus.

The development stages of pregnancy are called trimesters, or three-month periods, because of the distinct changes that occur in each stage.

Stages of Growth: Month by Month

Month 1

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 the growing embryo.

The placenta also develops. The placenta is a round, flat organ that transfers nutrients from the mother to the baby, and transfers wastes from the baby.

A primitive face will take form with large dark circles for eyes. The mouth, lower jaw, and throat are developing. Blood cells are taking shape, and circulation will begin. The tiny "heart" tube will beat 65 times a minute by the end of the fourth week. By the end of the first month, your baby is about 1/4 inch long – smaller than a grain of rice!

Month 2

Your baby's facial features continue to develop. Each ear begins as a little fold of skin at the side of the head. Tiny buds that eventually grow into arms and legs are forming. Fingers, toes and eyes are also forming.

The neural tube (brain, spinal cord and other neural tissue of the central nervous system) is well formed. The digestive tract and sensory organs begin to develop. Bone starts to replace cartilage.

The head is large in proportion to the rest of the baby's body.

By the end of the second month, your baby is about 1 inch long and weighs about 1/30 of an ounce.

At about 6 weeks, your baby's heart beat can usually be detected.

After the 8th week, your baby is called a fetus instead of an embryo.

Month 3

Your baby's arms, hands, fingers, feet, and toes are fully formed. Your baby can open and close its fists and mouth. Fingernails and toenails are beginning to develop and the external ears are formed. The beginnings of teeth are forming. Your baby's reproductive organs also develop, but the baby's gender is difficult to distinguish on ultrasound.

By the end of the third month, your baby is fully formed. All the organs and extremities are present and will continue to mature in order to become functional. The circulatory and urinary systems are working and the liver produces bile.

At the end of the third month, your baby is about 4 inches long and weighs about 1 ounce.

Since your baby's most critical development has taken place, your chance of miscarriage drops considerably after three months.

Month 4

Your baby's heartbeat may now be audible through an instrument called a doppler. The fingers and toes are well-defined. Eyelids, eyebrows, eyelashes, nails, and hair are formed. Teeth and bones become denser. Your baby can even suck his or her thumb, yawn, stretch, and make faces.

The nervous system is starting to function. The reproductive organs and genitalia are now fully developed, and your doctor can see on ultrasound if you are having a boy or a girl.

By the end of the fourth month, your baby is about 6 inches long and weighs about 4 ounces.

Month 5

You may begin to feel your baby move, since he or she is developing muscles and exercising them. This first movement is called quickening.

Hair begins to grow on baby's head. Your baby's shoulders, back, and temples are covered by a soft fine hair called lanugo. This hair protects your baby and is usually shed at the end of the baby's first week of life.

The baby's skin is covered with a whitish coating called vernix caseosa. This "cheesy" substance is thought to protect baby's skin from the long exposure to the amniotic fluid. This coating is shed just before birth.

By the end of the fifth month, your baby is about 10 inches long and weighs from 1/2 to 1 pound.

Month 6

Your baby's skin is reddish in color, wrinkled, and veins are visible through the baby's translucent skin. Baby's finger and toe prints are visible. The eyelids begin to part and the eyes open.

Baby responds to sounds by moving or increasing the pulse. You may notice jerking motions if baby hiccups.

If born prematurely, your baby may survive after the 23rd week with intensive care.

By the end of the sixth month, your baby is about 12 inches long and weighs about 2 pounds.

Month 7

Your baby will continue to mature and develop reserves of body fat. Your baby's hearing is fully developed. He or she changes position frequently and responds to stimuli, including sound, pain, and light. The amniotic fluid begins to diminish.

At the end of the seventh month, your baby is about 14 inches long and weighs from 2 to 4 pounds.

If born prematurely, your baby would be likely to survive after the seventh month.

Month 8

Your baby will continue to mature and develop reserves of body fat. You may notice that your baby is kicking more. Baby's brain is developing rapidly at this time, and your baby can see and hear. Most internal systems are well developed, but the lungs may still be immature.

Your baby is about 18 inches long and weighs as much as 5 pounds.

Month 9

Your baby continues to grow and mature: the lungs are nearly fully developed.

Your baby's reflexes are coordinated so he or she can blink, close the eyes, turn the head, grasp firmly, and respond to sounds, light, and touch. Baby is definitely ready to enter the world!

You may notice that your baby moves less due to tight space. Your baby's position changes to prepare itself for labor and delivery. The baby drops down in your pelvis. Usually, the baby's head is down toward the birth canal.

Your baby is about 18 to 20 inches long and weighs about 7 pounds.

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Fertilization of the egg with sperm generally occurs during the two weeks following the first day of your last menstrual period.

The week of pregnancy that you are entering is dated from the first day of your last period. This means that in the first two weeks or so, you are not actually pregnant - your body will be preparing for ovulation as normal.

This MNT Knowledge Center feature is part of a series of articles on pregnancy. It provides a summary of each stage of pregnancy, what to expect, and insights into how your baby is developing. Take a look at the other articles in the series:

First trimester: fertilization, implantation, week 5, week 6, week 7, week 8, week 9, week 10, week 11, week 12.

Second trimester:week 13, week 14, week 15, week 16, week 17, week 18, week 19, week 20, week 21, week 22, week 23, week 24, week 25, week 26.

You will also see introductions at the end of some sections to any recent developments that have been covered by MNT's news stories. Also look out for links to information about related conditions.

Week 3: Fertilization

You will ovulate (release an egg) around two weeks after the first day of your period (depending on the length of your menstrual cycle).


The fertilized egg (zygote) divides repeatedly as it moves down the fallopian tube to the uterus. First, the zygote becomes a solid ball of cells. Then it becomes a hollow ball of cells called a blastocyst.

In order for fertilization to occur, sperm must be ejaculated into the vagina through sexual intercourse or otherwise be inserted through the opening of the cervix to travel into the fallopian tubes.2,3

Once in the fallopian tubes, the sperm will penetrate and fertilize the egg.2 During the third week after the first day of your last period, your fertilized egg moves along the fallopian tube towards your womb.

Once united, the egg and sperm form a zygote, which contains 46 chromosomes - 23 from the female and 23 from the male, which will ultimately determine the genetic make-up of your child.1-3

What are chromosomes?

These chromosomes will determine the sex and physical characteristics of the fetus, and influence personality and intelligence.1,3

Chromosomes are tiny threadlike structures that each carry around 2,000 genes. Genes determine a baby's inherited characteristics, such as hair and eye color, blood group, height and build.

A fertilized egg contains one sex chromosome from its mother and one from its father. The sex chromosome from the egg is always the same and is known as the X chromosome, but the sex chromosome from the sperm may be an X or a Y chromosome.

If the egg is fertilized by a sperm containing an X chromosome, the fetus will be female (XX). If the sperm contains a Y chromosome, the fetus will be male (XY).

At 3 weeks pregnant, the fetus is the size of a pin head

The zygote's journey has only just begun, It will spend several days making its way down the fallopian tube, at which time it will develop into a morula, a ball of 12 to 15 cells and then into a blastocyst.1,2,4

The blastocyst, which at this point is rapidly multiplying, is a grouping of cells that contains an inner collection of cells. This blastocyst will ultimately develop into the embryo and an outer shell whose purpose is to provide protection and nourishment to the growing embryo.1,2

At this point, your future baby is still a cluster of cells measuring approximately .0019 inches, which is approximately the size of a pin head.3

During this time, it is important to speak with your health care provider regarding your current or desired exercise routine and your nutritional status. Increasing your consumption of folic acid and other vitamins may be recommended.3

Substances that can harm your growing baby and should be avoided include alcohol, illegal drugs, certain medications and foods. Caffeine and smoking should be discussed with your health care provider.

At this stage of pregnancy, there are very few expected symptoms however some women may have mild cramping and an increase in vaginal discharge during the ovulatory stage.3

Recent developments on fertilization from MNT news

Selective embryo technique could lead to improved IVF

In a new study published in the journal Biomicrofluidics, researchers from Taiwan reveal the creation of a new technique they say could eventually lead to more effective, cheaper in vitro fertilization for couples struggling to conceive.

Three-parent IVF: the benefits and the risks

Recently, UK members of parliament voted in support of legalizing mitochondrial donation - a form of IVF that could prevent severe genetic diseases being passed from mother to offspring. If the House of Lords votes in favor of the technique next month, the UK will be the first country to allow the creation of embryos from the DNA of three people. While supported by many, others say mitochondrial donation is a step too far for genetic engineering. We take a look at both sides of the argument and investigate the US standpoint on the procedure.

IVF embryos: whole genetic code can be scanned for mutations

Genetic scientists have used a new technique that allows the whole genome of IVF embryos to be scanned via the cells of 10 biopsies. The researchers say the testing is the first to be able to detect all the new genetic mutations that happen uniquely in an individual, as opposed to only those that have been handed down from parents.

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