Week by Week: Your Baby's Development- T1

Arryst Biosciences is proud to present "Week-by-Week: Your Baby's Development." Please use this utility to follow your baby's development throughout the weeks.

Week-by-Week: Your Baby's Development:

::: TRIMESTER 1 :::

Week 1 and 2:

Pregnancy is a 40 week adventure! This journey begins on the first day of your last menstrual period (LMP). The menstrual cycle [wiki link] is driven by changes in hormone levels. During the first stage the lining of the uterus is shed. This is experienced as flow from the vagina, often lasting 1-4 days. During the next 5-13 days the uterine lining begins to build up again and the next egg/follicle starts to mature in preparation for ovulation. At day 14, ovulation occurs. One or more eggs are released into the fallopian tubes. At day 15-28 the uterus prepares for possible implantation. If implantation does not occur within approximately two weeks the uterine lining again begins to shed.

At the end of week two, ovulation occurs. Sperm can survive in cervical fluid as long as 5 days, so if sperm are already present fertilization may occur soon after ovulation!

So the first two weeks are a bonus towards your 40 weeks since you have not yet ovulated!

Week 3:

The sperm and egg meet forming a zygote! Congratulations on your conception! The gender is determined at fertilization and depends entirely on the sperm which carries either an X or Y chromosome. If an X sperm joins the egg (always X), then the gender of the zygote is female, and if a Y sperm joins the egg, the zygote is male. After just 24 hours the newly formed zygote consists of a cluster of cells and will commence a one or two week journey through the fallopian tubes to the uterus.

Your future child is smaller than a tip of a pen!

You may not yet know whether or not you or pregnant but if you are planning a pregnancy consider researching prenatal vitamins which are taken during pregnancy.

Week 4:

Once the zygote arrives in the uterus it will implant into the uterine wall. Some women may feel some cramping or experience light spotting at this time.

Women may experience breast tenderness and their blood volume will increase. This extra blood requires your kidneys to work overtime and is thought to contribute to the extra bathroom trips many women experience. Women often feel fatigued since the heart is also working harder to pump the extra blood. Hormonal effects may also cause fatigue.

The cluster of cells begins to arrange itself into three layers during the next two weeks. The outer layer will form the skin, hair, eye lenses, tooth enamel, and salivary glands. The middle layer will form the blood and vascular system, muscle tissue, kidneys, and ureter. The inner layer will form the trachea, bladder, urethra, prostate, ear canal, liver, and pancreas.

Based on pregnancy symptoms some women may take an at home pregnancy test this week and discover the exciting and somewhat scary news! Congratulations! Begin looking for a doctor whom you feel comfortable with. You can choose a family practitioner or an OB/GYN (obstetrics/gynecology) doctor.

Week 5:

Normally you would begin menstrual flow at the beginning of this week so this is the time that many women may use at home pregnancy tests when they realize their period is “late.” Pregnancy tests check for human chorionic gonatropin (hCG) levels and claim to be quite accurate on the first day of a missed period. hCG causes an increase in estrogen and progesterone which triggers many of the early pregnancy symptoms that women experience.

Make an appointment with a doctor to confirm the at-home pregnancy test results with a blood test.

Morning sickness and nausea are common during the first trimester. Acupressure bracelets or certain foods (candied ginger, saltines, graham crackers, cheese, or nuts) may help depending on your personal preferences.

Women will continue to experience breast and abdomen tenderness. Fatigue will continue as your body adjusts to the hormonal influences and organs work harder. Nausea often continues for the first trimester. There is a lot going on inside of your body and it is amazing the rapid development that occurs from what started with a small cluster of cells.

The neural tube begins to form. This will develop into the nervous system containing the brain, spinal cord, hair and skin. The heart and primitive circulatory system form.

Hopefully you have found some prenatal vitamins by this time. Also remember to stay hydrated! Prenatal vitamins are important, and mainly contain increased folic acid and calcium compared to normal multi vitamins. Increased folic acid intake greatly reduces the risk of birth defects.

Blood begins to pump. Already there is a heartbeat! The embryo is in the shape of the letter C. In the middle area of the C the umbilical cord and organs develop. The beginnings of arm and leg buds also appear. The embryo is 1.5-3 mm from head to rump!

Make an appointment with your doctor to confirm the exciting news if you have not already done so.

Women will continue to experience breast and abdomen tenderness. Due to the increase in blood volume, your kidneys must work harder to filter it all and you will continue to make those extra bathroom trips! Fatigue will continue as your organs work harder and your body adjusts to hormonal fluctuations. Some women also notice an increase in saliva and/or swollen/bleeding gums.

Week 6:

The embryo begins to take a more familiar shape as arm and leg buds begin to lengthen. Eye spots appear which are indicative of eye lens development. Nostril holes develop. Intestines and pancreas begin to form. Brain development progresses and the heart continues beating.

Fatigue and nausea remain, as do tenderness in breasts and abdomen. Women often experience water retention and increased bathroom trips.

At your first doctor appointment you will receive a full physical with blood and urine testing. In some cases an ultrasound is done.

Your future child has already tripled in size and is about the size of a pea now!

Week 7:

During week 7 the face begins to take shape with ear holes, some pigmentation in the eyes, and tiny nostrils. Teeth are beginning to develop. Fingers and toes are forming as the limbs lengthen but there are no individual digits yet. The beginnings of elbows can be spotted. The digestive system (intestines) and stomach start to form. The circulatory system becomes more complex, the lungs begin to form, and the brain continues to develop.

Your baby is the size of a pencil eraser!

During this week you will continue to experience fatigue. Morning sickness does not just occur in the morning but any time of day. Eating well can help you get through the day. You may also be experiencing cravings and aversions to certain foods and smells.

Week 8:

If you haven’t already experienced pregnancy symptoms you may start this week with fatigue, nausea, tenderness of abdomen and breasts. Breasts will increase in size during this time as well. Remember to stay away from alcohol during your pregnancy. Others around you may notice your pregnancy glow. As blood flow increases your cheeks and face may glow. Some women can feel their enlarged uterus (about the size of a softball) when they press on their abdomens.

The baby’s cartilage and bone are forming. Fingers and toes are webbed. Eyelids are forming, the nose is beginning to protrude, upper lip is taking shape, and the tongue is beginning to form. The intestines are moving into place.

Your baby’s heart beat is beating very quickly at about 150 beats per minute! Your child is 0.61 inches (1.6 cm) long and weighs 0.04 ounces (1 gram).

Week 9:

Your face may break out thanks to the raging hormones. Fatigue remains normal so make sure you can get the time to relax and sleep extra. Limit strenuous exercise. Breast size may increase as much as an entire cup size. Some women experience increased vein size or stretch marks and itching. Some women also experience leaking. The breasts prepare for breastfeeding at the beginning of a pregnancy. Do some reading to prepare yourself but do not become overwhelmed and remember to stay positive.

The baby is beginning to move more although you can’t feel it yet. Their joints are developing. Fingers are beginning to develop fingerprints and can even curve around objects.

Your baby is 0.9 inches (2.3 cm) long about the size of a quarter, and weighs 0.07 ounces (2 grams)!

Week 10:

To counteract your pregnancy symptoms make sure you get plenty of rest, drink extra water, and get some gentle exercise. You could join a prenatal class at this time. You may be starting to show signs of your pregnancy.

If you haven’t seen your doctor yet, schedule that first visit! Your doctor will do a thorough physical and then expect to see you for shorter appointments once a month until week 32 when you will go in every two weeks and then every week after week 36.

The baby is now called a “fetus” since critical development is achieved. Now your baby can primarily focus on growing bigger. The baby has a large head and their irises are beginning to form. The placenta is beginning to function.

The baby is now 1.22 inches (3.1 cm) long and weighs 0.14 ounces (4 grams).

Did you know you can take the Early Gender Test NOW to determine the gender of your baby?! You have only just found out you are pregnant and may be experiencing some pregnancy symptoms. Share the exciting news with your family (maybe now they’ll understand your nausea and fatigue!), begin forming that close bond with your new little baby boy or girl, and start thinking about names earlier!

Week 11:

Pregnancy symptoms continue but may taper off beginning next week! Consider beginning Kegels and other pelvic floor exercises. You may begin to start noticing weight gain. Normal weight gain during a pregnancy may be 25-35 pounds, or 40-45 pounds for multiple births. You do not need to double your calorie intake. Increasing it by 1/6th is about all that is necessary. Remember to eat healthy and include plenty of protein, folic acid, calcium and iron in your diet!

At this point your baby’s fingers and toes are separating and hair and nails are beginning to grow. The genitals are beginning to develop and the organs are beginning to function. Amniotic fluid is beginning to accumulate. The baby’s intestinal wall muscles practice contractions.

Your baby is now 1.61 inches (4.1 cm) long and weighs 0.25 ounces (7 grams).

Week 12:

Pregnant women may have more vivid dreams. Frequent bathroom trips continue. Think about when to decide to tell your boss that you are pregnant. Many women choose to wait until the second trimester.

Your child’s vocal cords are developing this week. Their eyes are moving closer together, ears are shifting into position. The intestines move further into the body and the liver begins to function. Insulin production begins. Development continues in the nervous system but the brain is fully formed. The thyroid gland begins functioning as do some other organs.

For some women, nuchal fold tests are typically done between week 11 and 14 to check for down syndromes by ultrasound. You also have the option to do amniocentesis or chorionic villus sampling (CVS) at this time.

Your baby is now 2.13 inches (5.4 cm) long and weighs 0.49 ounces (14 grams).

::: TRIMESTER 2 :::

Week 13:

The high risk period or your pregnancy ends. Some of the fatigue and nausea subside and instead you may be feeling hungry and craving certain foods.

Sex during pregnancy will not hurt the baby. The uterine wall, amniotic sac, and amniotic fluid protect the baby. There is also a mucus plug closing off the cervix and uterus. Orgasm will not cause you to go into labor early although you may feel some uterine tightening. Only in very rare cases will your doctor ask you to abstain from sex.

Avoid too much sugar and try to eat small meals throughout the day. Fruits and veggies are great! Avoid food with mercury or Listeria [a list of foods that have sometimes caused outbreaks of Listeria is provided by the Center for Science in the Public Interest and includes: hot dogs, deli meats, raw milk, cheeses (particularly soft-ripened cheeses like feta, Brie, Camembert, blue-veined, or Mexican-style “queso blanco”), raw and undercooked poultry, raw meats, ice cream, raw vegetables, raw and smoked fish and the green lip mussel.] And stay hydrated!

Your child is beginning to practice breathing in the amniotic fluid. The eyes and ears continue to develop and move forward on the head. The neck is lengthening and hands are gaining function. The placenta is providing nourishment. Heartbeat is still fast and all twenty baby teeth have developed. Baby can now suck his or her thumb!

Your baby is now 2.91 inches (7.4 cm long) and weighs 0.81 ounces (23 grams), about the size of a lime!

Week 14:

Your uterus is now about the size of a grapefruit. Because of continued bathroom breaks and since you may be beginning to show, finding a comfortable sleeping position can be a challenge. Shopping for maternity clothes is now on the to-do list. Consider the purchase of a belly band which can provide support to the uterus as it gets larger. Heartburn, indigestion, and flatulence occur more often as your stomach loses space to the uterus.


Your doctor may recommend some different prenatal tests during the next few weeks. Alpha-fetoprotein Test (AFP) (Quad or Triple Screen) [wiki link] screen for neural tube defects.

Your child’s thyroid glands are producing hormones and their bones are hardening. If your baby is a girl, her ovaries are developing; if a boy, his prostate is developing. Baby still has transparent skin now covered in very fine hair called lanugo. Thumb sucking and a lot of wiggling and movement continue.

Baby measures 3.42 inches (8.7 cm) long and weighs 1.52 ounces (43 grams), approximately the weight of a letter!

Week 15:

Braxton Hicks contractions [wiki link] occur from week 6 of pregnancy but you may just now be becoming aware of them (although some first time mothers do not know this is what they are feeling until week 22-23). They are a way your body practices for labor. Normally there is no danger of preterm labor. If you experience mild discomfort, try changing your position or activity, take a bath, and drink more water. Reasons to call your doctor include: an increase in contractions (more than 4 per hour), vaginal spotting or bleeding, or excessively painful contractions (feel like really bad menstrual cramps). After you pass week 37 a call to your doctor is not required until the contractions are 60 seconds long and occur about every 5 minutes.

Your stomach has less and less room meaning you may switch to eating many small meals during the day since large meals can cause discomfort. Increased blood flow causes your heart to work harder and increased bathroom trips continue.

Some women claim to feel more scatterbrained and clumsy during this time. Although there is no scientific data supporting this, it is important to keep in mind a woman’s change in center of gravity which requires some adjustment by her body. She may be retaining water causing swelling in the fingers which may lead to some fumbling. Fatigue can also be a cause of forgetfulness.

You may experience round ligament pain [wiki link] which manifests as a strong pain the abdomen. This is caused by the uterus expanding and stretching the ligaments. Sometimes a change of position brings on the pain. A warm heating pad may help alleviate symptoms. If other symptoms occur contact your doctor.


Baby’s legs are now longer than his or her arms and a lot of squirming is going on inside of you. You may be able to feel baby’s movements for the first time! Baby still has very thin skin (blood vessels are visible through skin) and fingernails and toenails are growing. Eyebrows are developing and the ear bones are hardening.

Baby now measures in at 3.98 inches (10.1 cm) and weighs 2.47 ounces (70 grams) (about the size of a small apple or orange).

Week 16:

You should soon feel movement and eventually be able to distinguish kicks from hiccups. As your hormone levels stay high expect mood changes, dizziness, and headaches. Your breasts will enlarge and you may experience pain in your lower back and abdomen. Remember never to lift heavy objects. For a more comfortable sleeping position try sleeping on your left side with a lot of pillows surrounding your body to support you.

This may be a nice time to plan a relaxing get away. If this is your first child, your life is about to change forever! If this is not your first child some relaxation away from home and busy children may still provide an important benefit. Consult with your doctor before undertaking a major vacation.

Some women will have an ultrasound in the next few weeks. Ultrasounds screen for abnormalities or deformities and may also be able to tell the gender of your baby, although accuracy significantly increases after week 18. Your doctor will be taking measurements of the baby and not specifically looking for the gender. The American College of Obstetricians and Gynecologists (ACOG) discourages ultrasound for gender determination since it is unknown how ultrasounds affect the baby so fewer ultrasounds is generally assumed to be better.

It is also important during this time to choose the hospital where you plan on delivering.

Fat begins forming under baby’s skin to help maintain body temperature after birth. The ears move from the neck to the head and baby is beginning to be able to hold their head and neck straighter. The kidneys are functioning and bile is secreted into the stomach. Your baby depends on you for all of his or her nutrients. Scalp hair grows and may already have color. Facial muscles develop and your baby may even be able to open or close their mouth. A lot of sucking, swallowing, and blinking are occurring inside of you. Even though your baby lives in a liquid environment they are practicing breathing in there! Inhalation of small amounts of amniotic fluid is practice for the extrauterine environment.

Baby is 4.57 inches (11.6 cm) and weighs 3.53 ounces (100 grams). The baby and the placenta are about the same size at this point.

Week 17:

Increases in gestational diabetes [wiki link] have been in the news recently. Chances are your doctor is testing you for gestational diabetes at each check up. Gestational diabetes is diabetes which appears during pregnancy and can raise the risk of your baby also developing diabetes. Risk can be decreased if care is taken to regulate your blood glucose levels. Blood glucose levels should not have major spikes and dips but remain relatively constant. Take time to consider the foods that you are consuming to avoid blood glucose spikes. Eat many small meals throughout the day to avoid dips in blood glucose.

As the skin covering your breasts and abdomen stretch they may become itchy. Each week you have more chance that your back is going to begin to ache as your weight increases and your center of gravity shifts. Your gums may bleed (perhaps due to the increased blood flow and hormone levels). Increases in bodily secretions (sweating, nasal congestion, and/or vaginal discharge) are normal and will subside after birth. Some women develop linea nigra (a dark line from your belly button down) which disappears after birth. Sometimes freckles on your face and skin darken but this also disappears after birth. Remember to drink plenty of water and keep your feet up whenever possible to prevent swelling.

Your baby has a more normal human form in posture and appearance. Fingertips and toes develop pads. The eyes move forward on the face although they remain closed. The ears protrude from the head and by this time your baby can hear extrauterine voices and music. They are already familiar with your heart beat, breathing, and stomach grumbling. The umbilical cord thickens and there is accumulation in the bowels. Cartilage making up your child’s skeleton, transitions to bone.

Baby measures 5.12 inches (13 cm) long and weighs in at 4.97 ounces (140 grams).

Week 18:

A number of genetic tests are available. These tests do not tell you whether or not your child will develop a certain genetic disease but only inform you of the percentage risk of obtaining certain genetic diseases. Consult with your doctor about whether these tests are right for you.

A protective material forms on baby’s skin (vernix) and the placenta continues growth. Alveoli in the lungs develop as do vocal cords. The heart is now visible by ultrasound. Your baby’s senses continue to develop. You may notice that baby may be more active in response to certain sounds.

The baby is now 5.59 inches (14.2 cm) long and weighs 6.7 ounces (190 grams).

Week 19:

Although you can feel the baby move, other people normally can’t feel the baby move by laying their hand on your belly until week 28.

Now is an excellent time to schedule some childbirth classes if this is your first child or you would like to meet other expectant couples.

The myelin coating over the nerves is developing. Hair is growing and the permanent teeth form behind the baby teeth. Male genitals and the female uterus develop. Her ovaries even contain the precursors to eggs. The kidneys are producing urine which is secreted into the amniotic fluid. Your baby is swallowing and breathing this nutrient-filled amniotic fluid. Baby’s sleep patterns begin to become similar to a newborn’s sleep patterns.

Baby now measures 6.02 inches (15.3 cm) and weighs 8.47 ounces (240 grams) – that’s a little over 0.5 lb!

Week 20:

At this time many women have an ultrasound and confirm the gender of their baby! Accuracy is normally 80-90%. While ultrasounds are not required during an otherwise healthy pregnancy, studies have shown that they increase bonding with the baby. In some countries ultrasounds are offered at every visit.

This week your belly button may change from an “innie” to an “outie” as the uterus presses up against it. If your belly button gets irritated due to rubbing against clothing, try using a band aid or piece of tape over it.

Until the baby moves down into the birthing canal (usually 4-6 weeks prior to birth) you may experience some shortness of breath as there is less room for your lungs to expand.

Your baby’s period of rapid growth ends. Baby has a strong heartbeat and finds loud exterior noises startling. Their legs now have normal body proportions. Nerve cells continue developing for the senses. The female baby’s uterus continues to develop. You begin the process of transferring your immunities to the baby for the remaining weeks of pregnancy.

This week baby measures 6.46 inches (16.4 cm) and weighs 10.58 ounces (300 grams) (these numbers are approximations).

Week 21:

Congratulations you are over half way there!

If you are suffering from back pain talk with your doctor about back brace options which may be covered by your insurance.

Although some of your symptoms lessen, you may feel anxiety about labor or motherhood. Try some activities to bond with your baby such as reading aloud or listening to music together. Begin getting the nursery together.

The baby’s white blood cells form. The skin is opaque and swallowing is more common. The eyelids are still closed. The tongue develops as do the womb and vagina for baby girls. The baby’s sleep patterns become more consistent.

Baby is 10.51 inches (26.7 cm) long (now measured from crown to heel instead of crown to butt) and 12.7 ounces (360 grams).

Week 22:

Frequent bathroom breaks continue as does the back pain. Digestion is harder than ever with the loss of space the stomach experiences.

Libido may increase during the second trimester due to the increased blood flow and bodily secretions. Make time for intimacy with your partner.

Your child can hear talking, reading, and singing. The fingernails reach the end of the finger and their eyelids and eyebrows are complete. The brain is in a rapid growth stage. The liver is functioning by breaking down bilirubin. Male babies are forming primitive sperm and testosterone. The testes are moving into place.

Baby measures 10.94 inches (27.8 cm) long and 430 grams (about one pound!).

Week 23:

Your doctor may palpate your belly which involves pressing on your abdomen and determining the position of the baby and taking measurements. Your doctor may also check your cervix in order to determine risk of preterm labor. During this time and for the rest of the pregnancy, continue to get plenty of rest and try to sit and put your feet up a few times a day to take the pressure off your cervix and swollen joints.

Baby’s body proportions are almost normal now. The eyes are formed but do not have color yet. The pancreas is functioning and the middle ear bones are hardening. Since deeper male voices travel further, the baby can hear male voices better than female voices. If your child is born now they have a 15% chance of survival which only increases as each day goes by.

Baby measures approximately 11.38 inches (28.9 cm) and weighs 1.1 pounds (501 grams) now.

Week 24:

Be aware of premature labor risks especially in the summer when it is easier to get dehydrated. Dehydration contributes to the risk of premature labor. If your cervix measures more than 2.5 cm you may be placed on bed rest. Some women experience a whitish vaginal discharge during pregnancy and even in their normal cycles called leukorrhea [wiki link]. This is normal. If you are itchy and your labia are enflamed you may have a yeast infection. Consult your doctor before using over the counter treatments.

The baby is beginning to gain some weight and is officially viable this week, meaning that if you gave birth this week baby has some chance of making it with the help of modern technology. Taste buds form and lungs continue to develop as baby practices breathing. Lines on the palm develop and rapid eye movement (REM) begins to occur during sleep.

Baby measures approximately 11.8 inches (30 cm) and weighs 1.3 pounds (600 grams) this week.

Week 25:

You may be having trouble sleeping at night due to the increased bathroom breaks (as many as one an hour!), trouble finding a comfortable position, and anxiety about the upcoming changes in your life. It is important to drink plenty of water but you can use pillows and bend your knees upwards to support you while sleeping. The American Pregnancy Association (APA) recommends that you sleep on your left side to allow the major vein, the vena cava, which tends to be pushed to your right side (and even more so with pressure from your enlarged uterus!), to remain unrestricted. This allows optimal blood circulation for you and baby at night.

Some women experience increases in melanin on the cheekbones, forehead, and nose. These darker skin spots may fade after pregnancy. Remember to wear plenty of sunscreen and stay hydrated.

Baby’s spinal structures are forming and the lung blood supply is developing. The nostrils are opening and there is an increase in sensitivity around the mouth and lips. The baby is beginning to have swallowing reflexes and their dexterity is improving. Baby can wiggle fingers and toes!

Baby weighs approximately 13.6 inches (34.6 cm) and weighs approximately 1.46 pounds (660 grams).

Week 26:

You may be able to share baby’s movements with other people this week! Although you have been able to feel baby squirming around for some time, it may have been difficult for others to feel anything. You are finishing up the second trimester!

Baby’s lungs forms air sacs and begin secreting surfactant. The spine is strengthening and baby can now respond to touch! The retinas are formed and baby can open eyes and blink. Brain waves in response to hearing and sight are now detectable.

Baby measures 14 inches (35.6 cm) and weighs 1.7 pounds (760 grams).

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