Pregnancy week by week
In the first month of pregnancy, even before a mum to be knows she is expecting, huge changes are already under way in her body, and of course in the baby’s too.
We divide pregnancy into three trimesters to make it easier to distinguish each stage of development. It is exciting to know what is happening in mum’s and baby’s bodies at different points. So let us go week by week to see how baby develops and what the key milestones are in each period.
Keep in mind there is no such thing as a textbook pregnancy, so there may be stages where you do not fully recognise yourself. We have gathered the main and most common characteristics.
First trimester
This period covers the first three months and lasts up to week 13 of pregnancy.
The first 4 weeks are still “invisible”, but no less defining
An egg is released from the ovary into the fallopian tube around day 14 of the cycle to be fertilised. Yet the length of pregnancy is counted from day 1 of the last period, so you add two weeks to the date of ovulation.
Fertilisation creates a single cell zygote. It begins to divide rapidly, reaching the 16 cell morula stage, then by days 4 to 6 further divisions form the blastocyst. Although an ultrasound cannot show it yet, the cells are already differentiating to take on different roles.
On day 7, it begins to implant into the uterine lining, now ready to receive it. Further cell divisions form the embryonic disc, and a cavity called the amniotic cavity develops. This stage is crucial because baby’s organs begin to form.
Mum may not notice the new hormonal activity such as changes in progesterone production and alterations in the uterine lining. But it can happen that implantation and these early beginnings do have symptoms. You can read more about this in our summary The signs of pregnancy.
Month 2: from spotting the gestational sac to tiny finger movements
Week 5 is memorable for many mums because this is often when a medical examination confirms the pregnancy. Only the gestational sac is visible on ultrasound at this stage, but the scan also plays an important role in confirming successful implantation, so it is not worth postponing. If there is a higher risk of ectopic pregnancy, it may be advisable to consult a doctor even earlier.
At this time, the nervous system starts to develop, the heart tube appears together with the vessels that ensure circulation. The heart beats for the first time, and by week 6 it can already be seen and heard. The main organs are formed, but still have significant development ahead. Limb buds appear.
In mum’s body, blood volume and heart rate start to rise. Pregnancy symptoms often crop up for the first time now: nausea, headache, tummy ache, spotting, changes in smell and taste, irritability, breast tenderness and swelling, frequent need to wee and constipation. There may be vivid dreams and marked tiredness.
By week 7, the 7 mm embryo’s eyes, ears and nasal openings can be seen on the monitor. It already has right and left ventricles, and the kidneys are forming.
In week 8, tiny limbs and kidneys develop further. The little face is taking shape beautifully. The limbs and features that create the human form become increasingly recognisable. The bladder and spine are visible. The two lateral brain ventricles are formed.
Mum may first notice an increase in bust and waist measurement. Pregnancy becomes even more consciously felt on an emotional level.
By week 9, the face keeps taking shape. The digital rays in the limbs separate, and you can see the spontaneous movements of hands and feet. Tiny as they are, baby can clench the fingers into a fist and suck them. The external genitals appear, but baby’s sex remains a secret to us for now.
Month 3: rapid development of the embryo
By week 10, baby measures 27 to 35 mm and weighs about 4 grams. The eyelids fuse during this period and remain closed until around weeks 25 to 27. The most striking change this week is the disappearance of the tail. The free fingers and toes are formed, and all 20 milk tooth buds become recognisable.
In week 11, the first swallowing and bowel movements can be observed. Nails appear, the iris in the eye forms, and fingerprints develop on the fingertips.
Week 12 can be considered a milestone. From now on we call the little one a foetus, and we can observe truly adorable behaviours: grimacing, pulling faces, pouting, thumb sucking, sipping amniotic fluid. Reflexes are present.
Important tests take place this week: ultrasound, nuchal translucency screening, checking for the nasal bone, blood tests and the combined test, which screens for certain chromosomal abnormalities (Down’s, Patau, Edwards syndromes).
Mum’s weight gain typically starts around week 12. Appetite often increases, but instead of the misleading “eating for two” idea, mindful eating is recommended. If possible, cut back on refined carbohydrates such as white flour and sugars, and choose fruit, veg, seeds and healthier grains as your carb sources.
During this period, skin often looks radiant, and the rounding bump becomes visible. To help prevent stretch marks, it may be worth starting to care for the skin with natural oils.
Second trimester
For many, the most comfortable stretch follows, when mum feels more energised and shares her joy with others. Of course, there are times when you may still experience unpleasant symptoms in this period. Let your doctor know about these, though in many cases there is no need to worry, as we have said above, no two pregnancies are the same.
Most mums begin to feel baby’s movements during the second trimester.
From month 4, the smoother days of pregnancy begin
At the start of the second trimester, nausea and sickness stop for many, though they can persist until 20 weeks or beyond for some.
At week 13, baby measures 10 cm and weighs 30 grams. Eyebrows and fine hair start to grow. The rib lines become visible. Development continues throughout the body, the third and fourth brain ventricles form, the liver starts producing bile acids, and production of glycogen which is baby’s energy store begins.
During antenatal care, several measurements are tracked. These include BPD which is biparietal diameter, HC which is head circumference, AC which is abdominal circumference, and FL which is femur length.
In week 14, the ear continues to develop, and a week later baby is able to hear mum’s bowel sounds and heartbeat. Baby also senses the tastes of the foods mum eats. Eye movements begin, although the eyes remain closed.
The AFP blood test is done in weeks 16 to 17. It can flag certain abnormalities, such as neural tube or abdominal wall defects.
From week 18, definite wriggles can be felt from within. Baby is 20 cm long and weighs almost 200 grams. Meconium appears in the bowel. In the lungs, alveoli start to form. The skin is covered in fine hair known as lanugo.
In week 19, the vernix caseosa begins to develop, protecting baby’s skin. In girls, primordial egg cells mature in the ovaries and the uterus forms. Permanent tooth buds form behind the milk tooth buds.
This is when it is time for the second genetic ultrasound.
In month 5, we reach the middle of pregnancy
At week 20, the lungs and immune system continue to mature, and production of insulin and white blood cells begins.
In week 21, weight gain is the main focus and the organs keep developing.
By week 22, the uterus, ovaries and vagina have formed in girls. In boys, the testes descend from the abdomen towards the scrotum. A sleep and wake pattern can be observed in the baby’s days, and by the end of the second trimester the little one usually settles head down.
The ongoing movements allow mum to sense and observe baby directly, which is a real joy.
In month 6, the first tightenings appear
By week 23, baby is about 500 g and 29 cm long. There is lots of movement and sensitivity to touch. Baby sometimes hiccups. Mum feels this as little rhythmic jumps.
You may notice painless tummy tightenings from time to time. These are harmless, help the uterus grow and support circulation.
By week 24, baby is developed enough that, if born, there would already be good chances of survival.
Mum has a glucose tolerance test, which can screen for gestational diabetes.
From week 25, the foetus grows about 1 cm a week, moves actively, and the insides of the bones begin to harden.
In week 26, the foetal position can already be observed.
Mum may complain of leg cramps, and sleep can be more difficult. Getting enough magnesium helps relax the uterus and may prevent cramps. However, not all forms are absorbed equally well, so it matters which source you use. Ask your doctor about this.
By week 27, baby weighs around 1 kg, and the liver, lungs and immune system are working. Eyes open this week and baby can blink. Baby can perceive pain.
Third trimester
From week 28, the final stretch begins, which brings more challenges for the expectant mum. It becomes physically heavier, yet emotionally more exciting.
From month 7, baby would be viable if born
Lungs and brain are mature enough that, in the event of premature birth and with medical support, there is a very good chance baby can continue to develop outside the womb. Eyes open and close in line with sleep and wakefulness, and baby turns towards light. Often baby is already head down. As space decreases, movement is harder, but kicks feel stronger. Fat continues to build under the skin to prepare for life after birth.
By week 31, baby weighs about 1.5 kg. Baby sleeps a lot, so movement can be harder to track, but it is worth paying attention.
By month 8, the lungs are fully developed
At week 32, baby has mature lungs and one important task remains. That is putting on weight.
By week 34, baby will likely be in a head down, also called cephalic, position.
At 35 weeks, weight increases rapidly, reaching around 2.4 kg.
Month 9: from week 36 to birth
Baby now begins to engage in the pelvis. The body is well padded with fat, the skin smooth and soft.
By week 37, weight is around 3 kg. Practising breathing movements means baby swallows more amniotic fluid and hiccups frequently.
By week 39, all organs are developed. Length is around 49 cm, weight about 3.2 kg.
At week 40, baby is fully ready to be born.
During the third trimester, movement becomes harder, and new symptoms may crop up. These include digestive issues, back and joint pain, shortness of breath and cramps. The increasing weight puts strain on the spine and breathing. It can feel especially good to rest. Emotionally, the soon to be mum and dad are thinking about the birth, and have usually prepared everything to welcome the new little family member.
If there are still a few clothes missing, do not panic. That is exactly why we created ready made bundles, so you do not have to hunt down each piece one by one, frantically searching for natural, soft fabrics.
Conscious preparation for pregnancy and putting healthy living first help you cope more easily with the changes and are of inestimable value in supporting baby’s health. Vitamins and nutrients are needed from the moment you start planning pregnancy. There are excellent quality products to choose from today. Knowledge about pregnancy and birth, educational books, videos and prenatal exercise can help ease fears.
For some, it helps not to trawl the internet constantly, but rather to relax, read a good novel, and simply feel well to gather strength.
Wherever you are in your pregnancy right now, we would love you to share your experiences and feelings with us.
Sources:
NHS - Week-by-week guide to pregnancy
Tommy’s - Pregnancy week by week
