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Fetus in the first trimester of fetal development

The First Trimester of Pregnancy

Week 1 & 2 – Gestational Age


Your menstrual period has just ended, and your body is getting ready for ovulation. For most women, ovulation takes place about 11 – 21 days from the first day of the last period. During intercourse, several hundred million sperm are released in the vagina. Sperm will travel through the cervix and into the fallopian tube.


If conception takes place, the sperm penetrates an egg and creates a single set of 46 chromosomes called a zygote, which is the basis for a new human being. The fertilized egg spends a couple days traveling through the fallopian tube toward the uterus, dividing into cells ; it is called a morula. The morula becomes a blastocyst and will eventually end up in the uterus. Anywhere from day 6-12 after conception, the blastocyst will imbed into the uterine lining and begin the embryonic stage.

Week 3 – Gestational Age (Fetal Development – Week 1)


The embryo is going through a lot of basic growth at this time, with the beginning development of the brain, spinal cord, heart, and gastrointestinal tract.

Week 4 & 5 – Gestational Age (Fetal Development – Weeks 2 & 3)


Arm and leg buds are visible, but not clearly distinguishable. The heart is now beating at a steady rhythm. The placenta has begun to form and is producing some important hormones including hCG. There is movement of rudimentary blood through the main vessels. The early structures that will become the eyes and ears are forming. The embryo is ¼ inch long by the end of these weeks.

Week 6 – Gestational Age (Fetal Development – Week 4)


The formation of the lungs, jaw, nose, and palate begin now. The hand and feet buds have webbed structures that will become the fingers and toes. The brain is continuing to form into its complex parts. A vaginal ultrasound can possibly detect an audible heartbeat at this time. The embryo is about ½ inch in length.

Week 7 – Gestational Age (Fetal Development – Week 5)

At 7 weeks gestation, every essential organ has begun to form in the embryo’s tiny body, even though it still weighs less than an aspirin. The hair and nipple follicles are forming, and the eyelids and tongue have begun formation. The elbows and toes are more visible as the trunk begins to straighten out.

Week 8 – Gestational Age (Fetal Development – Week 6)


The ears are continuing to form externally and internally. Everything that is present in an adult human is now present in the small embryo. The bones are beginning to form, and the muscles can contract. The facial features continue to mature, and the eyelids are now more developed. The embryo is about 1 inch long and is the size of a bean.

Weeks 9 thru 13 – Gestational Age (Fetal Development – Weeks 7 thru 11)


At 10 weeks, the embryo is at the end of the embryonic period and begins the fetal period. The fetus has grown to about 3 inches in length and weighs about an ounce.  The genitalia have clearly formed into male or female, but still can not be seen clearly on an ultrasound. The eyelids close and will not reopen until the 28th week of pregnancy. The fetus can make a fist, and the buds for baby teeth appear. The head is nearly half the size of the entire fetus.


Lean More About the Other Trimesters of Pregnancy:


    Second Trimester

    Third Trimester

Fetal Development: Second Trimester



Home / Your Developing Baby / Fetal Development: Second Trimester

Fetus in 2nd trimester of fetal development

Weeks 14 thru 16 – Gestational Age (Fetal age – Weeks 12 thru 14):


The fetus’s skin is transparent and a fine hair called lanugo begins to form on the head. The fetus begins sucking and swallows bits of amniotic fluid. Fingerprints which individualize each human being have now developed on the tiny fingers of the fetus. Meconium is made in the intestinal tract and will build up to be the baby’s first bowel movement. Flutters may be felt in the mom’s growing abdomen, as the fetus begins to move around more. Sweat glands have developed, and the liver and pancreas produce fluid secretions. The fetus has reached 6 inches in length and weighs about 4 ounces.

Weeks 17 thru 20 – Gestational Age (Fetal Age – Weeks 15 thru 18):


The baby has reached a point where movements are being felt more often by the mother. The eyebrows and eyelashes grow in, and tiny nails have begun to grow on the fingers and toes. The skin of the fetus is going through many changes and begins to produce vernix at the twentieth week. Vernix is a white pasty substance that covers the fetus’ skin to protect it from amniotic fluid. A fetal heartbeat can be heard by a stethoscope now. The fetus has reached a length of 8 inches and weighs about 12 ounces.

Weeks 21 thru 23 – Gestational Age (Fetal Age – Weeks 19 thru 21):


Lanugo now covers the fetus’s entire body. The fetus is beginning to have the look of a newborn infant as the skin becomes less transparent while fat begins to develop. All the components of the eyes are developed. The liver and pancreas are working hard to develop completely. The fetus has reached about 10-11 inches in length and weighs about 1 – 1 ¼ pounds.

Weeks 24 thru 26 – Gestational Age (Fetal Age – Weeks 22 thru 24)- Beginning the third trimester:


If your baby was delivered now, it could survive with the assistance of medical technology. The fetus has developed sleeping and waking cycles and mom will begin to notice when each of these takes place. The fetus has a startle reflex, and the air sacs in the lungs have begun formation. The brain will be developing rapidly over the next few weeks. The nervous system has developed enough to control some functions. The fetus has reached about 14 inches in length and weighs about 2 ¼ pounds.


    First Trimester

    Third Trimester

Fetal Development: Third Trimester


Home / Your Developing Baby / Fetal Development: Third Trimester

Fetus during the third trimester of pregnancy

Weeks 27 thru 32 – Gestational Age (Fetal Age – Weeks 25 thru 30):


The fetus really fills out over these next few weeks, storing fat on the body, reaching about 15-17 inches long and weighing about 4-4 ½ lbs by the 32nd week. The lungs are not fully mature yet, but some rhythmic breathing movements are occurring. The bones are fully developed, but are still soft and pliable. The fetus is storing its own calcium, iron and phosphorus. The eyelids open after being closed, since the end of the first trimester.

Weeks 33 thru 36 – Gestational Age (Fetal Age – Weeks 31 thru 34):


This is about the time that the fetus will descend into the head down position preparing for birth. The fetus is beginning to gain weight more rapidly. The lanugo hair will disappear from the skin, and it is becoming less red and wrinkled. The fetus is now 16-19 inches and weighs anywhere from 5 ¾ lbs to 6 ¾ lbs.

Weeks 37 thru 40 – Gestational Age (Fetal Age – Weeks 35 thru 38):


At 38 weeks the fetus is considered full term and is ready to make its appearance at any time. As your baby becomes bigger, you may notice a change in fetal movement. If you notice a decrease in fetal movement, make sure to talk with your doctor. The fingernails have grown long and will need to be cut soon after birth. Small breast buds are present on both sexes. The mother is supplying the fetus with antibodies that will help protect against disease. All organs are developed, with the lungs maturing all the way until the day of delivery. The fetus is about 19 – 21 inches in length and weighs anywhere from 6 ¾ lbs to 10 lbs.


    First Trimester

    Second Trimester

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