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How Long Is Each Trimester And Why?

Getting pregnant is one of the dreams of many women. Having one’s own child is a joyous matter that many people look forward to. Pregnancy is a crazy time in a woman’s life, considering all the physical and emotional changes that happen within this short time. For around 9 months, changes occur continuously, meaning that not a single month is exactly the same as the next. For that reason, pregnancy may be tiring, but it is far from boring.

Medically speaking, pregnancy lasts for 40 weeks. The “trimester” method of measuring pregnancy makes it easy to identify which timeframe the woman is in. However, measuring by months – as in 3 months per trimester- is not exactly accurate. The difference between a certain month and the corresponding weeks may cause the mother to worry about why a certain change is not happening yet or why something is happening too early.

The changes that happen throughout pregnancy can be explained week-by-week or in a range of weeks. This is because some events happen over a range of weeks and months. Since women are not alike, some women have a difference in the week in which they experience something (such as listening to a heartbeat or feeling the baby kick). To prevent pregnant women from worrying about the wellbeing of their baby, the ultrasound examination they do at their routine checkup gives a near-accurate idea of the week the woman is in and how her fetus is changing accordingly. Here, we analyze the weeks of pregnancy and the parallel changes that happen during each week range.

15 First Trimester Week 1-3

The first 3 weeks of pregnancy are surprisingly a time when the woman doesn’t know she’s even pregnant. Pregnancy is calculated from the first day of the last menstruation. By the time a positive pregnancy test is seen or a doctor is visited, it is likely that about 4 or 5 weeks of pregnancy have already passed. Although implantation has not yet occurred, folic acid is strongly recommended at this point for any woman trying to conceive.

By week 2, ovulation happens, which means the egg leaves the ovary and gets ready for fertilization. Sperm reaches the egg and one of them attaches to it to form the zygote. The egg then shuts out all other sperm. By week 3 fertilization happens in the fallopian tube. This whole process usually goes unnoticed because conception does not have any signs. The usual known pregnancy symptoms do not kick in until implantation.

14 First Trimester Week 4-7

In the 4th week, implantation happens, and with it, all the hormones that lead to pregnancy changes start to rise. The wall of the uterus accommodates the zygote, sometimes leading to spotting of blood. However, if those spots are not seen, which is the case for the majority of women, it is likely that the hormones are not high enough to be detected. This could lead to a false positive pregnancy test.

At week 5, the fetus is about 1/16 to 1/8 inch long. It forms a C shape with a top, bottom, front and back. It continues to grow in the same shape and can be later seen in ultrasound examinations. By weeks 6 and 7, a woman already knows she is pregnant and must arrange to see her doctor for routine checkups. The hormones progesterone and human chorionic gonadotropin (hCG) rises at week 6 and women start experiencing nausea and fatigue. At week 7, the baby’s brain and eyes develop rapidly.

13 First Trimester Week 8-10

During week 8, the baby begins to develop webbed fingers and toes. The body begins to move, but it is too small to be felt. At week 9, the webbing in the fingers disappears and the heart is almost fully formed. Although the fetus gets its oxygen from the mother through the umbilical cord, subtle breathing movements happen at around week 10. Also, the eyelids begin to fuse so they protect the eyes.

At this time, the weight of the uterus and baby is not big enough to hinder movement, but the fatigue makes women unable to exercise or walk for long distances. The uterus doubles in size, becoming around the size of a tennis ball. Breast size increases and the woman may need to buy bigger bras. There is also loss of interest in sex, because of mood changes and exhaustion. The nausea and vomiting persist, so they affect appetite negatively.

12 First Trimester Week 11-12

A fun fact about week 11 is that the baby can suck its thumb, sigh, and move its head. The head of a fetus is bigger than the rest of the body. At this point, the head alone makes up about half of the whole body. Although genitals are beginning to take on a specific form, either male or female, they are still not visible in ultrasounds exams.

At week 12, the baby’s body becomes more upright than the curled position it was previously in. Around this time, many women experience problems in the gastrointestinal tract. Gassiness and constipation are the most common. It is a must to drink enough water to substitute for all the blood and amniotic fluid the body is making for the baby. The uterus is growing even more, pushing into the abdomen. This is the time when mothers-to-be start considering pregnancy pants, or those with elastic waist.

11 Second Trimester Week 13-15

Week 13 marks the beginning of the second trimester. The morning sickness associated with the first trimester subsides, possibly disappearing by week 14 or 15. In the womb, the baby’s nose and lips are completely formed, giving its face more distinctive features. The baby is now able to hiccup, a trait that helps to strengthen the diaphragm in preparation for future breathing.

The thyroid gland, pancreas, and gall bladder also form at this point, preparing for future hormones release. At week 14, the liver is able to produce bile and the pancreas produces insulin. By week 15, the soft hair known as lanugo covers the head. Interestingly, female fetuses move their mouths more than males during this week. The mother-to-be may begin to feel Braxton-Hicks contractions and see fluid leak from the breasts, because the milk glands are busy preparing for the production of colostrum – the nutrient-rich first food that provides newborn babies with a dense dose of immunity.

10 Second Trimester Week 16-18

At week 16, the fetus becomes quite active. It is able to grasp and play with the umbilical cord and kick at the amniotic sac. The mother feels heavier and heavier, thanks to the increasing amniotic fluid volume, increased blood volume and expanding breasts. By week 17, the skin is still thin, but brown fat begins to deposit in the baby’s body, which plays a huge role in metabolism and heat generation.

Because the second trimester marks the beginning of bone formation, calcium intake is very important at this point. The fetus begins to form teeth by week 18. Also, eye development is happening fast and the now the fetus’s eyes are sensitive to light due to retinal development. It is also now able to hear noises happening outside the womb and may respond to them by moving. Gender detection is possible now that the genitals have formed and can be seen in ultrasounds.

9 Second Trimester Week 19-20

During these two weeks, the fetus continues to explore by touching the amniotic sac and all the surroundings. Sweat glands form and follicles in the ovaries form if the baby is a girl. To protect the baby’s skin from the aqueous environment in which it is constantly submerged, a white, creamy substance known as vernix caseosa is formed and coats the skin. Starting now, the baby starts receiving immunity in the form of antibodies from the mother.

Week 20 marks the completion of half the pregnancy. By this time, the risk of miscarriage declines greatly, giving mothers-to-be a sense of peace and confidence. Back pain starts happening in vulnerable women, because of the weight gain. At this time, a mother starts planning things like the baby’s name, the birth plan and the desired childbirth classes. Things start to feel a little bit “final” although there is much more to go.

8 Second Trimester Week 21-23

During these weeks, the baby’s eyelids and eyelashes are fully formed. Taste buds begin to form on the tongue. The baby begins gaining weight as it will until the end of pregnancy. It also starts breathing in and out efficiently. Tooth development continues as her molars and canines form by week 22. Fingernails also form at this point, along with the functionality and specialization of certain body organs.

In baby girls, the uterus and ovaries are formed and in baby boys the testicles descend to the groin. The mother’s uterus continues to grow to adapt to the increase in the baby’s weight. Baby movements are critical at this point. If a mother notices that the movements are decreasing or that something is wrong, she needs to get checked by her doctor. She must also check her iron levels, considering the increased blood flow in and out of the placenta to nourish the growing baby.

7 Second Trimester Week 24-26

An interesting fact of this period is that the fetus starts developing sleep/wake patterns. The baby now has wrinkly skin, because the skin is being formed more quickly than the fat that gives it a straight texture. Fetuses now respond to talking and touching by kicking, which makes playing with the baby a lot of fun. It is a major hearing milestone for the baby. Eyelids start opening up.

The lungs start forming surfactant, which is the substance that allows lungs to inflate. More regular active and inactive periods are noticed and so are sleeping and waking ones. Alveoli, which are air sacs in the lungs, develop by the end of week 26. For such an interesting period, one thing may start disturbing the mother - her sleeping position. It is hard to find a sleeping position that is comfortable with the growing tummy, so, mothers-to-be might have to consider maternity pillows at this point.

6 Third Trimester Week 27-28

Week 27 is the beginning of the third trimester. At this point, with the help of incubators, the baby has an 85 percent chance of survival if it is born now. The amniotic fluid is reduced by half, because the baby takes up more space. This causes the mother to sense the motion more and possibly see her abdomen move if the baby kicks hard enough. Stretch marks increase as the mother’s whole body gets larger.

The baby is still gaining layers of fat and the bones are almost completely formed, although they are soft. Fetal weight increases rapidly and can be detected in routine checkups. Between week 28 and delivery, the brain will increase in size about 400-500 times. The baby weighs more than 2.5 pounds and can now taste and smell. The baby’s eyes can produce tears. Sucking and swallowing skills improve a lot during this week in preparation for future feeding.

5 Third Trimester Week 29-30

Week 29 is all about hormones. The baby’s adrenal glands produce a chemical that the placenta later transforms into estriol, which is a form of estrogen. Research suggests that estriol plays a role in the production of prolactin. Prolactin is the hormone necessary for effective breastfeeding, because it promotes milk production and milk duct formation. This process is likely to make breastfeeding successful, even in case the baby is born early.

This can cause the mother’s breasts to leak as it prepares colostrum. An exciting fact is that some researchers claim that the eyes are able to detect light very well at this point, to the extent that placing a flashlight over the bump can stimulate fetal movement. At week 30, the baby may still be in an abnormal position. However, this is no cause for concern because a lot will change between now and delivery. The baby moves around 10 times every couple of hours.

4 Third Trimester Week 31-33

The baby now gains a lot of weight, but does not increase that much in height. It is around 14-16 inches tall. By week 32, all the five senses are developed. Lanugo starts disappearing. The mother may now encounter more Braxton-Hicks contractions. It might get scary, but it does not necessarily signal that the baby is being born early. At the end of week 33, the baby weighs around 4.5 pounds and increases only one inch in height, reaching 17 inches.

At any time in this range of weeks, the baby’s head drops in preparation for delivery. This position also allows more blood to flow to the developing brain. A reassuring fact about weeks 31-33 is that babies born during this week range have an excellent chance of survival without any medical assistance. With most of the major organs fully formed and the breathing movements becoming more regular, there is not much to worry about.

3 Third Trimester Week 34-36

At week 34, the eyes synchronize with waking and sleeping. They open when the baby is awake and close when it sleeps. Speaking of sleep, the baby sleeps a lot during these weeks because it is spending a lot of energy forming an amazing number of connections in the brain. The baby also dreams during sleep.

The nervous and immune system are under construction. The body fat that is necessary for body temperature regulation is being added all the time. The chances of surviving if born go up to 99 percent. By week 36, the baby is growing even more, making it less easy for it to move. However, the mother should still be able to count more than ten movements a day. Some babies respond to sweet drinks. If the mother is concerned about the rate of motion, she should try drinking orange juice then lying on her side. The baby should kick in a while.

2 Third Trimester Week 37-38

At week 37, the baby clearly resembles a newborn. The baby is still breathing, but has less opportunity to practice kicking, given the decreasing amount of space available to it. The intestines begin building up meconium, which will be the baby’s first bowel movement soon after birth. A hormone known as relaxin begins to relax all smooth muscles in the body, to prepare it for labor. The water can break any day during this week range.

The brain is theoretically fully developed, but actually, the connections being formed in the brain continue to form even after birth, so there is no “end date” on that process. Previously, babies born after week 37 were considered full-term. Yet, that notion is changing, because some problems are being noticed in babies who are born before 40 weeks’ gestation. Other doctors argue that babies grow at different rates, which means that the 40-week mark is only an indicative one.

1 Third Trimester Week 39-41

By week 39, the baby is as big as a pumpkin. Its head and abdomen have the same circumference, because the body is catching up to the massive growth in the head. Pregnancy gets very tiring at this time, because the baby kicks into major organs to make room for itself. Under normal conditions, the baby settles into a head-down position while facing the mother’s spine, known as anterior presentation.

Most of the hair covering the baby’s body sheds before birth, but some of the vernix caseosa persists to protect against the first contact with the outside air. Although pregnancy is 40 weeks long, week 41 is often observed in women who want to deliver vaginally and have not yet experienced any contractions or water breaking. Some researchers believe that the baby sends androgens to the placenta as a chemical signal when they are ready to be born! Starting week 40, there is an intensive checking routine to detect cervical dilatation and effacement to predict how close labor is.

Sources: FitPregnancy.com, Parents.com, WhatToExpect.com, TheBump.com, BabyCentre.co.uk

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