The First Trimester Of Pregnancy Week By Week: Everything The Future Mother Should Know

Save Share

Planning a family is a complicated and, in some cases, also a long process. Those 280 days ( 9 obstetric or 10 calendar months) are an exciting and difficult period in a future mother's life. When a woman learns that she's pregnant, various books become her constant companions. She predictably wants to know everything that's happening to the baby and her own body: how she should react to global changes and, no less importantly, how to behave in order not to hurt the future child. So, we are going to tell you what to expect, week-by-week, during the first trimester, which will be useful even for experienced mothers.

From a zygote to an embryo

We all know that pregnancy is the period between the moment of conception up to the beginning of labor, that is to say, the first contractions. It occurs after the fertilization of the egg, which happens as a result of it encountering an active spermatozoon that turns it into a zygote.

Week 1

24 hours after the formation of the zygote, that is to say, the fertilization of the ovum, its division begins. The first week of pregnancy can be characterized by the path of the zygote from the ampullar part of the fallopian tube - where fertilization takes place in most cases - to the uterine cavity and its attachment to one of its walls.

Despite the fact that the future child hasn't even become an embryo, in this period its sex is determined depending on the chromosome set that the spermatozoon contributing to conception possessed ( X-girl, Y-boy).

During the first week of pregnancy, hormonal changes are almost undetectable, however, some women can experience:

-enlargement and sensitivity of the breasts,

-somewhat sensitive nipples,

-drawing pain in the lower abdomen more typical of the beginning of menstruation;

-little pink-brown discharge;


-changes in eating habits;

-mood instability;

-weakness similar to the symptoms of A.R.V.I.

In this period, diagnosing the pregnancy is actually impossible due to the fact that the level of HCG is still too low and can't be identified either by tests or ultrasound scanning. However, it's worth noting that the first method is quite effective in the case of IVF, as it allows controlling the success of the egg attachment process. A slight increase in the basal temperature can be the only symptom of pregnancy occurrence, provided that the measurements have previously been carried out regularly over a long period of time.  

An interesting fact to note: different chromosomes differ in both appearance and viability. So, the potential 'boy carriers' are larger when viewed under a microscope and at the same time, less resilient. Their life cycle is about 2 days. At the same time,  the fertilizing capacity of X-chromosome carriers reaches 7 days. Few people know that they can predict the future baby's sex depending on how many days after ovulation the conception took place. Nevertheless, this data can't be considered reliable.

The merging of the egg and a spermatozoon doesn't always result in the implantation of the zygote. The most common reasons for this are:

-early beginning of menstruation, as a result of which the egg leaves the uterine cavity together with the rejected endometrium;

-the presence of an inflammatory process;

-qualitative changes in the structure of endometrium layer in case of using certain types of contraceptives.

Even if it's not possible to confirm the pregnancy yet, but it is desirable or predictable, you should sleep well, eat healthy food, spend more time in the fresh air and increase your intake of folic acid from the first week. You should be careful when it comes to physical activity. If you have previously led an active lifestyle, then keep in mind that the first trimester of pregnancy is not the best time to change anything.

When it comes to restrictions, smoking and drinking alcohol are the most unfavorable factors for the development of the future child. Previously, experts didn't recommend abruptly giving up bad habits, considering it a stress for the body. However, recent studies prove that the reverse is true. Moreover, the myths that there is a safe amount of alcohol and that red wine is useful for pregnant women have also been refuted. Each cigarette and even the smallest amount of alcohol are bad for the baby.  

Week 2

If the zygote was implanted in the uterine wall, the division process continues, but the structure of the newly formed cells differ, as that's how the basis of the future fetus's organs and systems development is set. It is also worth noting that this period is more favorable from the point of view of regeneration capacity on all levels. This means that if, for some reason, there are certain damaged parts of tissues, they can be quickly replaced with no consequences for the baby's health.

During the second week of pregnancy, the woman's well-being is almost unchanged. The only thing that can indicate the onset of pregnancy is some swelling in the abdomen caused by the increased secretion of progesterone. In general, the above-mentioned symptoms will remain the same with the only difference being that lower abdomen discomfort may get worse.

Finally, the good news is that during the second week of the first trimester, it's possible to find out that the future baby is growing in the mother's body. However, this can be done only by blood analysis, as HCG concentration is still too low in the urine.

Week 3

Probably, every woman knows that most unprompted miscarriages happen during the period of 3-6 weeks. The active division of the fetal egg's cells contributes to the new tissues becoming  highly specific and very vulnerable.

Among the most dangerous factors for the future embryo that can cause the development of various deficiencies, we can distinguish the following ones:



-excessive heat loss;




The woman's state of health undergoes considerable changes. The manifestation of symptoms typical of pregnancy explains the increasing amount of progesterone, which is simultaneously produced by the placenta being formed and functioning as an endocrine organ. One of the most obvious symptoms is drowsiness. It's very important to rest as much as possible-the future mother needs to sleep at least 8 hours a night.

At this time, the zygote's disc acquires a cylindrical shape, one of its ends somewhat increases and becomes more round- this is the future head. The baby forms a primitive intestine which eventually evolves into a digestive tract.

The beginning of the embryonic period

As a rule, the woman learns about her pregnancy due to the blocking of the endometrial layer rejection mechanism since menstruation doesn't start at the due date. It is at this very time that the fetal egg is developed enough to be considered an embryo.

Week 4

At this stage, the future baby's tissues begin to actively form. It already has anlages of spine, brain and spinal cord, intestines and heart and its length is 1.5 mm. Pregnancy symptoms become more and more pronounced but do not cause much discomfort yet. At this point, pregnancy can be diagnosed with the help of any modern method, including express tests.

At this period, it is necessary to be extremely careful about your health and well-being, as it is still critical.

Week 5

At the next stage of growth, the embryo's heart tube begins to contract. At first, it's irregular but soon the frequency of rhythmic movements reaches 120-132 beats a minute. It already has a head, inside which there are distinguished sections of the future brain, limbs, anlages of the main organs and systems.

Both height and weight increase a little- 2.5 mm and 0.4 g respectively. In spite of the fact that the future baby is too small, many women notice a slight increase in the volume of the abdomen. This is connected with the relaxing effect of progesterone on muscle tone. The most unpleasant phenomena characteristic of this period are toxicosis, which can and should be fought against, and the decrease of motility in the intestinal region, as a result of which constipation occurs.

Week 6

At the sixth week of pregnancy, the gynecologist can determine the increase of the uterus which resembles the size of a plum. Most often, it is at this very time that the future mum gets registered for a women's consultation.

The embryo is undergoing the following changes:

-the hemispheres of the brain become clearly delineated;

-the heart is already 4-chambered (consists of 2 ventricles and 2 atria);

-anlages of hearing and balance organs start to form;

-it moves, but the future mother doesn't feel it yet.

Also, the pregnant woman's body begins to undergo significant changes:

-the breasts increase in size to some extent, as they are preparing for lactation;

-the skin changes and as a result the woman needs to choose the right means to take care of it (dry skin often becomes ideal, while oily skin is more prone to rashes);

-vaginal discharge remains transparent but can become more abundant (it's important to remember that the change of discharge colour and appearance of unpleasant sensations in the perineum are a reason to immediately consult a doctor who will prescribe a safe treatment).

Week 7

The embryo is becoming more and more like a baby. Its length is already 13 mm and the upper part of the body is more developed, while the tiny tail is still in its place.

In this period, the following processes begin:

-the formation of hands and feet;

-initiation of milk teeth;

-the development of the skeletal system and thymus gland that will perform the function of the immune system;

-the synthesis of hormones by the adrenal glands, as a result of which the embryo starts reacting to stress and external stimuli.

The future mum should increase her intake of calcium-containing products, since it is this very micro-element that directly takes part in the growth of her future baby. Among unpleasant phenomena that a pregnant woman can encounter, the following ones are the most common:

i-ncreased drowsiness and fatigue;

-mood swings that should be avoided to prevent the baby's increased sensitivity to stresses;

-exacerbation of smell;

-thickening of the walls of veins if you already have a predisposition to varicose veins;

-impaired intestinal motility;

-abdominal distention;


-frequent urination.

Week 8

At this period, the head and body of the future child are the same length and its weight is about 3 g. It starts to move its small hands and legs, the face starts forming eyes covered with eyelids, ears, nose and lips. All the bones and joints are already formed. Moreover, at this point of time, the active process of central nervous and respiratory systems' development begin and the heart valves of the aorta and pulmonary artery appear. The baby is already beginning to react to touches, sweat glands appear on the skin. Although the baby's sex is still impossible to determine, the future girl has her eggs laid.

Week 9

At this stage of development, the fetus begins to grow at a faster rate and at the end of this period, the length is already 30 mm and weight is about 4 g. The head is still disproportionately large and tightly pressed to the chest, whereas the back has straightened a little and the tail has turned into a coccyx. The frequency of heart contractions reaches 150 beats per minute, the baby actively moves, but it's still imperceptible.

The baby becomes more susceptible to stress, as at this period the middle layer of the adrenal gland begins to be laid. Also, the process of lymph gland formation begins and the thyroid gland is already fully functioning. During a medical examination, it is possible to distinguish the features of the face, feet and elbows.

Mom undergoes the following changes:

-active weight gain begins (if toxicosis doesn't prevent it);

-a slight increase in the volume of the abdomen is noticed;

-a venous net is developing on the breasts.

During this period, the risk of developing anemia is great. This is caused by the change of hemoglobin concentration, therefore it's important not to neglect the necessity to get registered and be regularly observed by a certified specialist. In spite of the constant increase in the amount of blood, its composition is renewed unevenly. Furthermore, the consumption of oxygen considerably increases, that's why it's worth being in the fresh air more often.

Week 10

At this stage, the baby has already grown up to 35-40 mm, but is still freely floating inside the uterus, periodically moving spontaneously and from time to time pushing away from its walls. The hands and legs become longer, the process of buttock formation begins, the head becomes a bit round. By the 9th week, the body is already covered with fluff, and taste and auditory receptors begin to function.

Mum must necessarily gain some weight and will notice an increase in the volume of the abdomen. If this hasn't happened, it's necessary to consult a certified specialist in order to minimize the manifestations of toxicosis or to correct the diet. In some cases, women also notice the appearance of pigment spots. As a rule, they are temporary and disappear without external interference.

From an embryo to a fetus

At around the 11th week of development, the future baby already possesses the anlages of all the necessary organs and systems, which means that its not an embryo anymore.

Week 11

Height can reach 44-60 mm, and weight is around 8 g and it becomes more and more mobile. The grasping reflex is actively developing, but the hands are still a little longer than the legs, the feet of which are already actively working. Tiny nails appear on the fingers of the fetus, the iris appears in the eyes and more than 250,000 neurons appear in the brain per minute. Very soon, it will also be possible to determine the sex of the baby, as the reproductive organs are already forming. Blood vessels can be seen under the skin and the heart rate reaches up to 160 beats a minute.

Mum's state is somewhat stabilized at this period:

-manifestations of toxicosis decrease;

-due to drowsiness and increased fatigue, it is still recommended to regulate sleep and vigilance;

-there might be a wish to eat something inedible (for example, chalk), which is often a sign of anemia;

-instability of hormonal background and psychoemotional state are manifested stronger.

Week 12

This period is the last one in the first trimester. The fully functioning placenta completely satisfies the needs of the baby, who already has the anlages of all the systems and whose appearance is becoming more like that of a small human being- eyelashes, ear laps and eyebrows are already formed. The excretory system is gradually starting to function- from time to time, a small amount of urine gets into the amniotic fluid, the intestines contract. Although the movements of the fetus are still involuntary, they are already more active and boys start to produce testosterone.

At around this time, mothers notice some changes in their state:

-swelling of the breasts, which is connected with the increase in the volume of glandular tissue;

-gradual growth of the abdomen caused by the fact that the uterus no longer fits into the pelvic cavity and begins to protrude above the pubic bone;

-changes in skin pigmentation which disappear over time after the delivery;

-more abundant vaginal discharge, the colour of which should be transparent or slightly whitish.

Women should take care of their health state and well-being throughout pregnancy. At the 12th week, it's necessary to register for a women's consultation, if you still haven't done it, and undergo a complete medical examination. The latter includes an ultrasound which is done in order to determine the position of placenta and the state of the mouth of the womb, the measurements of the fetal collar zone in order to identify possible abnormalities in its development, as well as genetic screening.

It is during the first trimester that the key processes are set in a woman's body and they later determine the parameters of pregnancy development and its normal course. The baby goes through a difficult path from a zygote to a fully formed little human being and this whole process requires great efforts and constant control from both the baby and the mother. And after this period, there comes the time when every woman can really enjoy her new unique state.

Source: obstetrician-gynecologist Lilit Nasibyan

This article is solely for informational purposes. Do not treat yourself, and in all cases consult a certified healthcare professional before using any information presented in the article. The editorial board does not guarantee any results and does not bear any responsibility for harm that may result from using the information provided in the article.

Save Share
Very useful

9 months ago


Health Tips for Women
1.2 k Subscribers

Home Mia App Top Women's Health Cycle & Periods Fertility Issues Love & Relationship Pregnancy & Parenting Fitness & Nutrition