Pregnancy week by week — weeks 1 to 40

Your baby will grow and change dramatically week by week, forming rapidly into a little person. I have listed the various stages so you can track the changes and see what's going on inside you and how you are creating another human being - a miracle of Nature. What happens week by week will vary slightly between each pregnancy. Let's start at the beginning with how your baby was conceived.

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Conception

A man's semen meets your egg in the fallopian tube during a natural conception, where dozens of sperm attack the egg, eroding down the outer layer allowing one sperm to enter. Conception happens and a flash of white light occurs called the `zinc spark'. The egg is now an embryo and travels down the fallopian tube over five days to the uterus (see figure 3 on the next page).

(4 days) Ovary

Mature Ovulation Sperm follicle Fertilisation

Uterus lining Corpus luteum (6 days)

Figure 3. Fertilisation, journey and implantation of the embryo First Trimester

Week 1

The embryo (blastocyst) loosely attaches to the uterus wall around six days after fertilisation. On day seven the embryo releases enzymes that allow it to burrow more firmly into the outer layer of the uterus called the endometrium. Once the embryo has burrowed into the outer layer of the uterus wall, the endometrium is then referred to as the decidua. The decidua separates from the uterus during labour, much like during menstruation (7).

Week 2

The outer layer of the embryo (trophoblast) forms into two layers (syncytiotrophoblast and cytotrophoblast), providing the embryo with nutrients and creating the outer sac and the placenta. The embryo releases human chorionic gonadotropin (hCG), which a pregnancy test picks up and tells you that you are pregnant. Levels of hCG peak around week 9 (see table 2, page 21) and maintains the sac that once held the egg (corpus luteum), allowing it to continue releasing progesterone until the placenta takes over at around week 8-12.

A small cavity appears which enlarges to become the amniotic cavity. The amniotic cavity fills with amniotic fluid, which comes from your blood. Later the baby will contribute to the amniotic fluid from its urine. The amniotic fluid acts like a shock absorber for your baby and helps to regulate the baby's body temperature and prevents him or her from drying out or from getting stuck to surrounding parts. The amniotic fluid ruptures before birth and is known as waters breaking (bag of waters) (7).

Around eight days after fertilisation, the yolk sac forms, which supplies nutrients during the second and third week of development and is the source of blood cells that will form into the reproductive organs and part of the gut. Around 12 days after fertilisation, the chorion forms, which surrounds your baby. It protects the baby and becomes part of the placenta. It also protects the baby from your immune system by blocking antibody production and promotes the production of T cells that suppress normal immune responses. By the end of the second week, chorionic villi begin to develop. They are finger-like and connect the embryo to the uterus wall. Blood capillaries within the chorionic villi will connect to the heart via the connecting stalk, which will be the future umbilical cord.

Week 3

The third week sees the start of rapid development with the three primary germ layers that lay the groundwork for future organ development in weeks four to eight. The three primary germ layers form the endoderm, mesoderm and ectoderm. Each of these layers will grow to be different parts of the baby's body. The inner layer, called the endoderm, will form the digestive tract, urinary bladder, gall bladder, liver, pharynx, hearing, tonsils, larynx, lungs, thyroid gland, pancreas, thymus, prostate, vagina, urethra, sperm and eggs. The middle layer, called the mesoderm, becomes the heart, spleen, kidneys, blood and lymphatic vessels, reproductive organs, part of the eyes and skin, muscles, cartilage and bones. While the outer layer, called the ectoderm, becomes the brain and nervous system, the eye lenses, tooth enamel, mouth and nose, hair, skin, nails, pituitary and pineal gland and the nervous system.

During the third week of development, small spaces appear that merge to form a larger cavity that will divide the embryo into two parts called the splanchnic mesoderm and the somatic mesoderm. The splanchnic mesoderm forms the heart, pericardium, blood vessels and the muscles of the respiratory and digestive organs. The somatic mesoderm forms the bones, ligaments, blood vessels and the connective tissue of the arms and legs.

Week 4

Your baby is now the size of a pinhead. During the fourth week of development, the embryo undergoes very dramatic changes in shape and size, moving from two-dimensional to three-dimensional and tripling in size. During this process, a head fold and a tail fold form. The head fold develops into the heart and mouth, while the tail fold forms the anus. The head end of the neural tube develops into the primary brain vessels. The skeletal muscles of the neck, trunk and limb form together with the connective tissue including the dermis of the skin, vertebrae and ribs.

The foetal pole forms, where the heart will start to beat. The placenta formation begins and will take over the role of progesterone manufacture from the corpus luteum from around weeks 8-12. The limb buds develop. Eyes and ears begin to develop, as does the tail and body systems. Your cervical plug forms from cervical fluid and seals the head of the cervix creating a protective barrier that prevents any foreign substances from entering the womb that could cause an infection and harm the baby.

Week 5 (2mm/0.1in)

Your baby has gone from a circular celled embryo to looking like a tadpole with a tiny tail. There is rapid development of the brain, which makes the head look disproportionally bigger than the rest of the body. The baby's nervous system is developing and the foundations for its major organs are in place. As the ectoderm develops, a groove forms and the layer of cells folds to form a hollow tube called the neural tube. This will become the baby's brain and spinal cord. At the same time, the heart forms as a simple tube-like structure. Your baby already has some of its own blood vessels and blood begins to circulate.

Week 6 (4mm/0.2in)

Your baby is now the size of a pea. By week six, the head continues to grow even larger and the limbs develop substantially more. Their heart now has four chambers and starts beating at 110 beats per minute (bpm). The neck and trunk begin to straighten. Your baby still looks like a tadpole. The head is called the crown and its bottom called the rump. Measurements of the baby's growth are taken from the top of the crown to the bottom of the rump (head to bottom), as the legs grow later. The heart can sometimes be seen beating on a vaginal ultrasound scan at this stage, although not always. It depends on the location of the embryo and the equipment used, so try not to worry if you go for an early scan and they cannot find it, that can happen. The developing arms and legs become visible as small swellings called limb buds. Little dimples on the side of the head will become the ears, while small black dots on the face will become the eyes and a small bump on the head will form into the nose.

Week 7 (1cm/0.4in)

Your baby's brain is growing rapidly at an amazing rate of 100 cells a minute. This rapid growth rate requires a lot of energy from the mother, hence why a lot of pregnant women can feel tired at this time. During this period, it is important to eat, sleep and rest as much as possible. Their eyes and ears continue to develop. Your baby's mouth and tongue are being formed, as are its kidneys. The limb buds start to form cartilage, which will develop into the bones of the legs and arms. The arm buds become longer and the ends flatten out with digits forming that will become fingers. The heart beats faster at 120bpm. Nerve cells continue to multiply and develop as the nervous system (the brain and spinal cord) starts to take shape.

Week 8 (1.6cm/0.6in)

This week is the final week of the embryonic period. By the end of this week, your baby will start to look more human and less like a tadpole. During this week, the fingers grow and change from being webbed to singular, the tail becomes shorter and disappears, the eyes develop, eyelids come together and the external genitals begin to form. Your baby has grown a lot and is now the size of a bean.

Week 9 (2.3cm/0.9in)

Your baby is now called a foetus, which is Latin for `offspring'. This is the first hurdle reached. Well done! Their heart is now beating at 170bpm. The legs are lengthening and forming cartilage. Your baby starts to look more human-like and their nose, eyes and spine develop further. Your baby is still inside its amniotic sac and the placenta is continuing to develop. At this stage, your baby still gets their nourishment from the yolk sac.

Their face is slowly forming and their eyes are bigger and more obvious and have some colour in them. The mouth and tongue are more formed and the tongue starts to form tiny taste buds. Their hands and feet are developing - ridges identify where the fingers and toes will be. The major internal organs (such as the heart, brain, lungs, kidneys and intestines) continue developing. From weeks 8-10, you may see your midwife for the first time. During that meeting they will ask you lots of questions relating to the pregnancy and check when your last period was and give you your expected due date.

Week 10 (3.2cm/1.2in)

Your baby's bones and cartilage are forming and small crevasses on the legs and arms will form into the knee and elbow joints. Fingerprints are being formed. Their ears are starting to develop on the sides of your baby's head and inside the head their ear canals are forming. The jawbones are developing and already contain all the future milk teeth. Their heart is now fully formed and beats at 180bpm, much faster than yours. You may hear this on a Doppler machine. Do not be alarmed by the rapid heartbeat as it is perfectly normal. Your baby is making small, jerky movements, which may be seen on an ultrasound scan. Your baby is already producing urine from their operational kidneys, which is excreted into the amniotic fluid. If your baby is a boy, his testosterone levels will surge from now until week 18. A Harmony test (see page 36) can identify the gender of the baby from this time.

Week 11 (4.2cm/1.6in)

Your baby is growing quickly and the placenta is rapidly developing (it will be fully formed at around week 12). The bones of the face are now formed. Hair follicles start to form. Their ear buds look more like ears as they grow. Your baby is starting to have breathing movements. They can open their mouth and swallow. The baby's head makes up one-third of its length, but the body is growing fast. The fingers and toes are separating and nail beds are starting to form.

Week 12 (5.3cm/2.1in)

This is the second major hurdle reached. Well done! All your baby's major organs, muscles, limbs and bones are in place and begin working together. The immune system, hormones and digestion start functioning. Your baby starts to make random movements. Your baby begins to concentrate iodine in its thyroid and produce thyroid hormones at about this time. The pancreas is beginning to make insulin and the kidneys are producing urine. The skeleton is made up of tissue called cartilage and starts to develop into hard bone. At this time you may have your first hospital scan, which will check the heartbeat, size, spine, organs, etc. and make sure everything is okay. You can stop your acupuncture sessions now if you want to. However, if you are still anxious or have a history of repeated miscarriages, are over 40 or used a donor egg, then I would recommend carrying on (see Chapter Seven).

Your breasts have probably increased in size and may feel uncomfortable in your usual bras. Breasts can enlarge by 25-50 per cent (4). Have your breasts measured for a new maternity bra and find one that does not contain a wire, for comfort and support.

Week 13 (7.5cm/2.9in)

Your baby's ovaries or testes are fully developed inside their body and the genitals are forming outside their body. Where there was a swelling between the legs, there will now be a penis or clitoris growing, although usually you will not be able to find out the sex of your baby using an ultrasound scan at this stage. Your baby's vocal cords begin to form.

Second Trimester

Week 14 (8.7cm/3.4in)

Your baby's neck becomes longer and they begin to swallow little bits of amniotic fluid, which pass into the stomach and out as urine. Body hair starts to grow. Eyes and ears move to their final places. Toenails start to appear.

Week 15 (10.1cm/4in)

Your baby will start to hear its first sounds, which will be your heartbeat and voice as well as muted sounds from the outside world. Even though your baby's eyes are closed, they may register a bright light outside your body as their eyes start to become sensitive to light. Your baby moves more and may be sensed, called quickening, although most mothers do not feel their baby moving until later, which is normal.

Week 16 (11.6cm/4.6in)

The muscles of your baby's face can now move and the beginnings of facial expressions appear. Eyes begin moving but remain closed. Eyebrow hair and eyelashes are formed. Your baby's nervous system continues to develop, allowing the muscles in their limbs to flex. Your baby's lungs continue to develop and hearing improves.

Week 17 (13cm/5.1in)

During this week, your baby grows quickly. The body grows bigger, with the head and body becoming more in proportion. The face begins to look much more human. Your baby's mouth can open and close. The lines on the skin of the fingers are now formed and your baby already has his or her own individual fingerprints. Fingernails and toenails are growing. Your baby practises sucking and swallowing in readiness for your breast milk.

Week 18 (14.2cm/5.6in)

Your baby can move around quite a bit and may respond to loud noises from the outside world, such as low-frequency music (bass). You may not feel these movements yet, especially if this is your first pregnancy. If you do, they may feel like a soft fluttering sensation. Your baby may start to yawn when sleepy and hiccup as their digestive system develops. You might feel these hiccups too. Around this time, your baby becomes covered in a very fine, soft hair called lanugo. The purpose of this is thought to keep the baby warm. The lanugo hair usually disappears before birth and forms their first poo as they will eat and digest it.

Week 19 (15.5cm/6.1in)

Your baby's ears, nose and lips are now recognisable. Your baby carries on swallowing parts of the amniotic fluid that they are suspended in and gets a taste for what you have been eating that day. This helps your baby's digestive system practise for when they are born.

Week 20 (25.5cm/9.9in)

This week sees a big jump in size of your baby. At this time you may have your second hospital scan, where they will perform a thorough check of your baby's organs, size and make sure everything is okay. You should now be able to tell the sex of your baby from an ultrasound scan, if they are in the correct position on the day of the scan. The baby is covered in a substance called vernix caseosa, a greasy white protective layer that protects them from the amniotic fluid. This coating is shed before birth at around week 40, although some babies are born with it. Your baby has some scalp hair and is capable of producing its own antibodies.

Week 21 (27.2cm/10.6in)

Your baby is now able to suck and grasp and may have bouts of hiccups. Your baby is beginning to get into a pattern of sleeping and waking, which will not be the same as yours. These patterns of waking will often remain the same when they are born and wake to feed. When you are in bed at night, feeling relaxed and trying to sleep, your baby may wake up and move about. This is because when you are still, the baby notices this and wakes up. When you are active and moving around the baby finds this relaxing and sleeps, hence why new parents often put their baby in a buggy, sling or car to get them to sleep, as movement helps them to relax.

Your baby will now weigh more than the placenta. The placenta will keep growing throughout pregnancy, but not as fast as your baby. Your baby's arms and legs are now in proportion and they will be moving them around, which you may feel.

Week 22 (28.8cm/11.2in)

Your baby carries on growing and developing. Their internal organs and brain develop more. They have a better sense of the outside world and might react to loud noises. Going to the cinema or places that play loud music may wake them up and get them moving.

Week 23 (30.4cm/11.9in)

Your baby starts putting on weight and begins to fill the skin they are in. You may notice more and more movements and get a better idea of their sleep patterns. Your baby will begin to have rapid eye movements during sleep at around this time.

Week 24 (32cm/12.5in)

Due to advances in Western medicine, your baby is now viable and can survive if they are born at this time. Your baby is putting on more weight from growing bones, organs and body fat.

Week 25 (33.6cm/13.1in)

Your baby's eyelids open for the first time and they will soon start blinking. However, it is not until some weeks after birth that your baby's eyes take on their natural colour.

Week 26 (35.1cm/13.7in)

By now your baby's heart rate will have slowed to around 140bpm. Your baby's brain, lungs and digestive system carry on maturing and their nostrils start to open.

Week 27 (36.5cm/14.2in)

Your baby carries on developing and becomes more reactive to the outside world with noises, lights and the foods that you eat.

Third Trimester

Week 28 (37.9cm/14.8in)

Your baby's lungs are almost mature and operational. Your baby carries on practising sucking, swallowing and digesting until they are born. During this week, you may see your midwife to check your blood pressure, urine for protein, the size of your bump and to give you vaccinations, such as the whooping cough. Your baby has eyelashes and its skin is red in colour.

Week 29 (39.3cm/15.3in)

Your baby continues to be very active at this stage and you may be aware of lots of little movements.There is no set number of movements you should feel each day and every pregnancy is different. The sucking reflex is developing by now and your baby may start to suck their thumb or fingers. Your baby is growing plumper and their skin begins to look less wrinkled and much smoother.

Week 30 (40.6cm/15.8in)

Your baby's brain develops and starts to control and take over from things such as regulating its body temperature.The white, greasy vernix and the soft, furry lanugo (fine hair) that has covered your baby's skin begin to disappear. Your baby's eyes can now focus.

Week 31 (41.9cm/16.4in)

Your baby's brain continues to grow and develop.They carry on putting on weight and practising moving their arms and legs. Their wake and sleep patterns become more regulated. Make a note of these as they will often be the times they wake and need feeding once they are born.

Week 32 (43.2cm/16.8in)

By around now, your baby is usually lying with their head pointing downwards, ready for birth. This is known as a cephalic presentation. If your baby is not lying head down at this stage, it is not a problem as some babies move later. The amount of amniotic fluid in your uterus is increasing and your baby is still swallowing fluid and passing it out as urine. Your baby is forming muscle and storing body fat. If your baby is a boy, his testicles are descending.

Week 33 (44.4cm/17.3in)

Your baby's brain and nervous system are fully developed. Your baby's bones are also continuing to harden, apart from the skull bones. These will stay soft and separated until after the birth, to make the journey through the birth canal easier as they can overlap each other allowing the head to reduce in size. Your baby is curled up in your uterus, with their legs bent up towards their chest. They are starting to rapidly gain weight and you may feel this extra weight. This will mean less space in your uterus for them and you may see them kick or poke on the surface of your belly. Your stomach and bladder may have less space as a result of your baby's rapid growth.

Week 34 (45.6cm/17.8in)

Your baby's toenails and fingers are fully formed. If your baby is a boy, his testicles have now descended from his abdomen into his scrotum. Your baby's brain grows and matures. Your baby has around 8 per cent body fat.

Week 35 (46.7cm/18.2in)

From around now, you may be aware of your uterus tightening from time to time.These are known as practice contractions (Braxton Hicks) and are a normal part of pregnancy (see page 14). Your baby may now be head down (engaged) and getting ready for delivery. When this happens, you may notice that your bump has moved down a little and you feel more pressure on your pelvis, which might make it more uncomfortable to walk. This is normal. Sometimes the head does not engage until later or when labour starts. If your baby is breech then you can start moxibustion treatment from now. Otherwise, an external cephalic version (ECV) can be performed at around 36-37 weeks to turn a breech baby (see page 197).

Week 36 (47.8cm/18.6in)

By now, your baby's lungs are fully formed and ready to take their first breath when they are born. Their digestive system is now fully functional and ready to receive your breast milk. You can restart acupuncture treatment now if you stopped at week 12. Research has shown that having acupuncture treatment as part of your pre-birth preparation can help to normalise labour by needing less pain relief medication, less chance of being induced and less time spent in hospital.

Week 37 (48.9cm/19.1in)

Your pregnancy is now termed early term. Your baby's digestive system now contains meconium - the sticky, green substance that will form your baby's first poo after birth. It may include bits of the lanugo (fine hair) that covered your baby earlier in pregnancy. The lanugo that covered your baby's body is now almost all gone, although some babies may have small patches of it when they are born. Mothers older than 40 or who are at high risk, for example those that conceived using in vitro fertilisation (IVF), may be medically induced around now, although it depends on your personal circumstances, obstetrician and hospital.

Week 38 (49.9cm/19.5in)

Your baby carries on shedding their protect layer called vernix and the lanugo hair. Fat levels carry on increasing as your baby gets ready for the outside world. Your baby now has around 16 per cent body fat. The baby's skin changes from red to its natural colour.

Week 39 (50.9cm/19.8in)

Your pregnancy is now at full term (280 days). Your baby's growth has slowed down now and if its head was not engaged before it should now be. At this point, most mothers feel uncomfortable day and night due to the size of their bump and feel more pressure on their pelvis. You should now feel regular practice contractions (Braxton Hicks, see page 14).

Week 40 (51.2cm/20.2in)

Your baby is now full-sized and is the size of a small pumpkin. Although newborn sizes can vary, the average baby weighs 3.2kg (7lb). Your baby's hair and fingernails will be getting longer. Some babies with long fingernails can pierce the membrane they are in causing your waters to break. Let's now look at what happens if you go over 40 weeks.

After week 40

If you go past your due date, see going over 40 weeks for what to expect and how natural and medical induction methods compare; see also acupuncture for labour induction.

My Pregnancy Guide

My Pregnancy Guide by Dr (TCM) Attilio D’Alberto is a comprehensive week-by-week guide to a healthy pregnancy and labour, based on over 750 peer-reviewed research studies and 20 years of clinical experience. It blends the latest evidence-based science with the proven theories of traditional Chinese medicine to give you everything you need for a confident, well-supported pregnancy and a positive birth experience.

The book covers pregnancy week by week, an optimal pregnancy diet and supplements, how to reduce your baby’s risk of developing autism, managing complications including gestational diabetes and pre-eclampsia, preparing for labour, pain relief options, acupuncture for labour induction, natural herbs to support birth, and postnatal recovery. Available in paperback, Kindle and ebook from Amazon, Waterstones and all major bookshops.