Development of your baby

1st Trimester

The 1st trimester refers to the first third of your pregnancy, the first 13 weeks. It is the period in which the baby is conceived and develops.

Week 1

The gestational age of your pregnancy is calculated from the first day of your last period, since traditionally this date has been known far more accurately than the date of conception.  Therefore, at 'week 1' you are not yet pregnant at all; but the countdown to delivery has already begun!

Week 2

The Ovum (egg), which has been developing in your Ovary, is released as a result of hormone changes, and travels along the Fallopian tube, where it is fertilised by a sperm towards the end of 'week 2'. About 30 hours after fertilisation, the single cell divides into 2, and then 4, and then 8, and continues to divide rapidly into a growing cluster of cells, known as a 'Morula' as it continues on its passage to the Uterus (womb).

Week 3

On reaching the uterus, the cell cluster; which has taken the form of a fluid-filled sphere, called a 'Blastocyst', with an inner layer that is your developing baby and an outer layer forming the placenta and amniotic sac (the protective bag of water surrounding the baby); burrows into the soft lining of the Uterus, using fine hairs to anchor and absorb nourishment. The Blastocyst already contains a full set of DNA from you and your partner, which determines the baby's sex, eye colour, and other traits.

Week 4

The cell collection has now developed sufficiently to be officially classed as an 'Embryo' and is about the size of a poppy seed or grain of rice. During this time, the inner cells form two, and later three, layers that will become different parts of your developing baby's body: the brain, nervous system, skin, eyes and ears; lungs, stomach and gut; and heart, blood, muscles and bones. At present it is still too small to be seen on Ultrasound, but over the next six weeks all of your baby's organs will begin to develop and some will even start to function.

Week 5

At only three weeks after conception your baby's heart is forming and the nervous system begins to develop. The embryo forms into a shape reminiscent of a sea-horse, with head and tail ends, although it's only the size of a sesame seed. The neural tube, which will eventually form into the spinal cord and brain, runs from the top to the bottom of the embryo. A bulge in the centre of the embryo will develop into your baby's heart. At this time, the placenta develops. It is through the placenta and its finger-like projections, called 'Chorionic Villi', that an embryo receives nourishment from its mother.

At week 5, this is the very earliest that ultrasound can be used to visualise the pregnancy; (5 - 8wk Early Pregnancy Viability Scan) and even then a trans-vaginal ultrasound examination will be required and the baby itself is too small to be seen.

Week 6

The embryo has now formed a well-defined head and body; the brain is rapidly growing and the heart begins to beat. Optic vesicles, which later form the eyes, begin to develop this week on the sides of the head, as do the passageways that will make up the inner ear. 'Limb buds' indicate where the arms and legs are growing, and amazingly miniscule buds for milk teeth begin forming in the jaw, and nostrils start to develop. When measured by ultrasound, the baby's length, from the top of its head to the base of its bottom (Crown Rump Length) measures about 4mm.

Week 7

Your baby has now grown to the size of a sunflower seed, with an oversized head bent over the chest. Tiny lips, nostrils, eyes beneath the skin and dimple ears are forming. The major internal organs are in full development and a primitive brain has been created. Tiny paddle-shaped hands and feet have formed on the end of the limb buds. The umbilical cord has now formed, and will soon become essential to provide your baby with food and oxygen, and to remove waste.

Week 8

The face is becoming recognisably human as the jaw, mouth, tongue, nose and eyelids form. Tiny fingers and webbed toes are appearing. The fetus is now moving, although its small size - about that of a kidney bean - means you won't feel it yet. The baby's intestines are getting longer, but there isn't enough room for them all in the fetal abdomen, so they protrude into the umbilical cord.

At week 8, we can undertake an ultrasound examination to confirm the pregnancy and estimate the Gestational Age; (8 - 13wk 1st Trimester Dating Scan) as the baby is now big enough to be clearly seen and measured.

Week 9

The baby's head is taking on a slightly more upright appearance as the neck forms and the back straightens The small tail on the bottom of your baby's spinal cord has shrunk and almost disappeared;  arms, legs, fingers and toes are all growing longer and the limbs are now able to move individually. These movements can be seen on Ultrasound scans, but the baby is still too small for them to be felt just yet.

Week 10

The ankles and wrists are clearly defined, and the eyes have developing beneath the fused eye-lids. The fetal brain continues to grow at a phenomenal rate of about 250,000 new neurons (nerve cells) per minute. The webbed fingers and toes are separating and all of the vital organs have now been formed

Week 11

The placenta now begins its work in earnest, supplying the baby, via the umbilical cord, with the food and oxygen rich blood necessary for its continued development and growth. The head, which is still oversized and accounting for about half the total body length is now held erect away from the chest. The external genitalia begin to develop.

Week 12

The fully formed baby has doubled in size in the last three weeks, at about 5cm length; and can stretch, roll, yawn, flex its fingers, drink the amniotic fluid and empty its bladder. The internal organs will continue to mature, genitals continue their development. You can now hear your baby's heartbeat at a prenatal check-up.

Week 13

Your baby now weighs about as much as a 50 pence coin! If your baby is a girl, nipples appear and her uterus, cervix and vagina form. If your baby is a boy, a penis forms. Even as early as 13 weeks, your baby may find and suck their thumb.

At week 13 onwards, we can undertake ultrasound examination to monitor the pregnancy (13 - 34 wk Reassurance Scan). NOTE: Although the genitals are now formed, it is too early to identify the fetal sex by ultrasound.




Read about Fetal Development in the Second Trimester


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