Month by month pregnancy guide

Pregnancy is divided into three trimesters (three month periods). This division is convenient, particularly in describing the symptoms the woman may experience.

First Trimester (0-12 weeks)

At 4 Weeks

The embryo

By the time a pregnancy reaches the 28th day, or the end of the 4th week, it is firmly embedded in the lining of the uterine cavity and is just visible to the naked eye. Extremely important changes are taking place in the woman’s body at this early stage, although she may be unaware of her pregnancy until she misses her first period.

The embyro is about half an inch long and already has a recognisable head, nervous system, body and the beginning of minute limb buds. Towards the end of the fourth week, the heart begins to beat, although it is still not visible on an ultrasound scan.

The mother

It is usually around this time that the woman begins to suspect she may be pregnant, when she misses her first period.

At 8 Weeks

The embryo

The embryo measures about one and a half inches. Vital organs continue to be formed, and the digestive, respiratory and urinary systems are taking shape. The eyelids have formed, but will not open for several more weeks. The ears, ankles, fingers and toes are barely discernible at this stage, and it is around this time also that the part of the ear which is responsible for balance and hearing (the middle ear) begins to grow.

The circulatory system has been established and the heart is beating strongly. The lungs, which are formed as tiny, solid organs on each side of the mid-line at around seven weeks, continue to grow rapidly, but remain solid at this stage.

The head of the embryo is still very large in proportion to the body, although the face is beginning to assume a discernible shape with depressions present where the eyes are destined to be, and a definite shape to the nostrils. The two sides of both the upper and lower jaw have fused as well, so that the mouth can be recognised.

Shoulders, elbows, hips and knees are all beginning to become apparent and the limbs continue to develop rapidly. Since the spine is fully formed at this stage, the embryo is able to make very tiny movements, although the mother will be unaware of these movements for many weeks yet.

The mother

The woman at this stage of pregnancy may develop so-called morning sickness (the nausea and vomiting may happen at any time during the day). She may also notice fatigue, frequency of urination and tender and enlarged breasts. Many women also develop lower abdominal pains at this time, these are due to stretching of the ligaments and muscles of the pelvis. There is often a creamy white discharge from the vagina.

At 12 weeks

The foetus

By this stage, the rapidly growing pregnancy is no longer medically referred to an embryo, but graduates to the status of a foetus. It is now about three-and-a-half inches long and could be said to have reached a major milestone as all the vital organs have formed and are beginning to function.

It is during the critical period before the end of the 12th week that congenital abnormalities may occur. However, it is reassuring to note that, once an organ has been properly formed, it cannot come to any great harm.

The kidneys of the growing foetus begin to secrete urine by the end of the 12th week and soft nails have formed on the ends of the fingers and toes. The buds which will eventually form 20 milk teeth are already in place in the gums and the sex organs have developed, although they cannot yet be seen on an ultrasound scan.

The face is properly formed and the head begins to take on a more rounded appearance but is still bend forward. The internal ear is properly formed and the external ear continues to assume its distinctive shape. The foetus continues to flex his limbs, but no movement is yet felt by the mother.

The mother

The uterus is now enlarged by the pregnancy and can be felt in the lower abdomen as a very soft swelling arising out of the pelvic cavity. Other physical changes, which are apparent to the mother at this stage, are changes in the size and shape of the breasts. There is very definite breast enlargement and an increase in the size and number of veins on the surface of the breasts. Small, raised pink nodules known as Montgomery's tubercles also form on the areola, which is the area of pink, delicate skin surrounding the nipple.

The formation of these tubercles (which are not visible in a non-pregnant woman) can very often be one of the most reliable early signs of a first pregnancy. They are not as reliable a diagnostic sign in subsequent pregnancies, however, as they do not disappear completely after delivery.

Morning sickness usually settles down somewhere after the 12th week and very rarely lasts for the duration of the pregnancy.

Second Trimester (13-28 weeks)

At 16 weeks

The foetus

The foetus continues to grow and develop rapidly. All limbs are properly formed and the joints are moving vigorously. Fingers and toes have assumed their proper shape and the nails are fully formed. Although the head continues to be quite large in proportion to the size of the body, there is rapid growth in the body at this stage. All the primary sex characteristics are in place and the sex of the foetus is obvious to even the untrained observer. The baby’s face has matured to the extent that he can smile and frown.

A fine downy hair (called lanugo) covers the entire surface of the body, including the face. The eyebrows and eyelashes start to grow at around 16 weeks.

The mother

It is during this second trimester (three to six months) that many women are at their peak both physically, mentally and emotionally. Their body has adjusted perfectly to being pregnant and the nausea which they may have been experienced in the first 12 weeks has disappeared.

At 20 weeks

The foetus

Hair appears on the head of the foetus, who is now increasing rapidly both in length and in weight. Since it still has a relatively large amount of amniotic fluid (the fluid inside the womb) in which to move about, he is able to move around and rotate with ease and to exercise his limbs very vigorously. He can also suck his thumb.

The mother

Sometime between the 16th and 20th week, most pregnant women will become aware of their baby’s movements for the first time. The medical term for this phenomenon is ‘quickening’ and it is regarded as one of the major milestones of pregnancy.

From about the 12th week of pregnancy, the uterus moves up out of the pelvic cavity and continues to enlarge at a regular rate so that it reaches the umbilicus (belly-button) by around week 22. A very definite ‘bump’ is present.

At 24 Weeks

The foetus

Although all the vital organs are now fully formed and are functioning, the foetus would find it difficult to maintain independent life for any length of time if it is delivered prematurely. This is because the lungs are not yet properly matured. However, there are many cases where babies born at 24 weeks have survived with very high levels of intensive nursing care in premature baby units.

By the time he reaches 24 weeks, the foetus measures over 12 inches in length and weighs about one-and-a-half pounds. It has a normal amount of muscle on the legs and arms but still looks very thin as deposits of subcutaneous fat (which gives babies that chubby, cute appearance) have not yet been laid down. The foetus is aware of sounds around him such as music and the voice of his mother.

At 28 weeks

The foetus

At 28 weeks, the foetus is now legally viable, which means that it is capable of a separate, independent existence and must be registered if it is born prematurely.

The foetus is now covered in vernix, which is a greasy, cheese-like material covering the whole of the skin from head to toe. The purpose of the vernix is to prevent the skin from becoming waterlogged due to its continued immersion in the amniotic fluid in the womb.

Since the baby’s body has grown more than the head over the past number of weeks, the head is now only very slightly out of proportion to the size of the body.

The eyes begin to open and close and the baby continues to move very vigorously at this stage. This movement may feel like sharp thumping on the abdominal wall, and will be interspersed by long periods of quiet which should not give rise to any anxiety in the mother as the baby must rest sometime!


Third trimester (29-40 weeks)

At 32 weeks

The foetus

The foetus is now perfectly formed and the head is in proportion to the body. If born prematurely at this stage the chances of survival are put at more than 95% with intensive nursing care in a high-support premature baby unit. In Ireland, these services are only available in the larger maternity hospitals.

A large amount of vernix continues to cover the entire body of the baby and while deposits of subcutaneous fat continue to be laid down, he remains relatively thin.

It is at this stage that the baby usually settles in an upside-down position within in the womb in preparation for birth. Although the baby continues to move vigorously, these movements will not always be so apparent to the mother as his space within the womb is becoming very limited indeed.

The mother

The expectant mother has now entered the third trimester of her pregnancy and this is the time when she is likely to experience feelings of tiredness and discomfort due to her rapidly expanding girth and the unprecedented growth of her baby. She may complain of the mechanical problems associated with the growing pregnancy — low back pain, constipation, urinary frequency and ankle swelling. Varicose veins may become more prominent at this time. Stretch marks may become visible on the tummy.

At 36 weeks

The baby

The countdown begins! With only four weeks to go, the baby is now fully mature and has assumed that characteristic chubby, cute appearance of a new baby due to the laying down of large amounts of subcutaneous fat. While there is more than a 95% chance of survival if the baby is born at this stage, the main reason why some babies fail to survive is due to the insufficient development of the lungs.

The baby is still covered in a large amount of vernix, and sometimes this vernix is also present in the amniotic fluid.

The mother

The mother may be "slowed down" at this time because of the size of the bump. She may also complain of the mechanical problems associated with the pregnancy (see above). She will notice increasing contractions of the womb muscle (Braxton Hicks contractions).

In over 50% of women having their first baby, the head descends (or engages) into the pelvis at this stage in preparation for the birth. This is referred to as ‘lightening’ and usually results in a considerable relief of pressure in the upper abdomen of the mother. It is quite significant when it occurs, but there is no cause for concern if it fails to happen.

At 40 weeks

Medically speaking, this marks the end of a normal pregnancy, but it must be remembered that this is just an average, not a hard and fast rule. Many pregnancies last less than 40 weeks, while some women go into labour as late as the 42nd week!

During the final four weeks of pregnancy, the rate of increase in the baby’s weight is likely to average about half a pound per week, so a baby born at 36 weeks is likely to weigh two pounds less than if he were born four weeks later.

At 40 weeks, the baby is still covered in a large amount of vernix, which may be particularly thick on the skin creases around the groin area, armpits, behind the knees and around the neck. The only parts of the body where the vernix is not present is around the mouth and on the eyes. The fine downy hair (called lanugo) which covers the entire surface of the baby’s body from about 16 weeks, has almost disappeared by the end of the pregnancy, but may still be present over the shoulders and on the arms, legs and occasionally across the forehead.

Babies are always born with blue eyes, although the colour of the eyes may change within minutes of delivery. The fingernails and toenails protrude exactly to the ends of the fingers and toes, but not beyond. They are also very soft, so they cannot damage the delicate skin by scratching during the first 24 hours of life.

The average weight of a full-term baby is eight pounds. Babies who weigh less than five-and-a-half pounds at birth are described as ‘low birthweight babies’ and are likely to need intensive nursing care in the first few days of life.

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