First and second week of pregnancy: embryo development and symptoms

First week of pregnancy

During the 1st and 2nd weeks of pregnancy, although technically you are not pregnant yet, you can already consider yourself at the beginning of the wonderful journey of pregnancy. You are «beginners», having your first child, you probably don’t know yet what the 40 weeks of pregnancy have in store for you, and you are looking for information to follow the growth of the embryo and then the fetus in an informed and conscious way, and even better understand the symptoms you will experience in the next 9 months .

So let’s start right away by telling you that those that are conventionally considered the first 10 – 15 days of pregnancy are the most anomalous, and are also the most “subjective”, above all because in these days the very first symptoms of pregnancy are not yet present. . The first week of pregnancy coincides with the arrival of your last period, when you are not pregnant yet.

It might therefore seem strange to you that these first two weeks are considered the period of pregnancy, but it is a convention adopted internationally and has its advantages in the relationship between gynecologist and patient, as we will see in more detail in the following paragraphs.

The onset of pregnancy

The conventions relating to the dating of the day of delivery

As mentioned, by convention  the due date is calculated by adding 280 days to the first day of the last menstrual period (LMP). This is a rough estimate, in fact only 4% of women give birth at 280 days, and only 70% within 10 days of the expected date. 

This approximation arises mainly from the fact that the real date of conception, which occurs with the fertilization of the ovum, is unobservable.

Even if they are rather approximate, it is preferable to refer to estimates of the beginning of pregnancy (starting from the LMP, as seen above, or from an ultrasound).

Another source of variability in the estimate of the due date is related to the normal variation in the rate of fetal maturation, and in the moment of birth. The possibility that there is a normal variability of these phenomena is plausible, even if it is little discussed in the literature.  

Estimation error and natural variability are indistinguishable without an exact measure of gestational age. Thus, characteristics that have been associated with length of gestation may be due to errors in estimation of gestational age, natural length of pregnancy, or both. Without an exact measure of gestational age it is impossible to separate them.

Although the natural date of conception in humans is not directly observable, there is evidence that conception occurs within 24 hours of ovulation. So the day of ovulation around the time you tried to conceive can be used to mark the actual start of a pregnancy .

Several studies carried out on attempts at natural conception have tried to estimate the duration of gestation based on ovulation. 

All of these studies assessed ovulation using the basal body temperature method, which however was found to be an inaccurate method. Furthermore, none of these studies explored maternal or pregnancy characteristics that might be associated with gestational length.

A recent study, based on statistical data, concluded that the duration of pregnancy depends on several factors, including:   

  • the duration of any previous pregnancies, which can be considered as indicative of the typical gestational duration of the individual woman;
  • some events that take place in the first two weeks after conception, thus suggesting that the “trajectory” of pregnancy is somehow defined in the very first days after conception.

In the rest of our discussion we will refer to the pregnancy dating convention which provides,

  • that the pregnancy lasts about 280 days, or 40 weeks;
  • that the count of days and weeks of pregnancy begins with the first day of the last menstrual cycle (LMP).

This is perfectly in line with what is commonly referred to in our country in the relationship between gynecologist and patient.  

First signs of pregnancy during the first and second week

No two pregnancies are alike and have the same symptoms occurring at the same time.

This is even more true in the first days after conception, which occurs between the end of the second and the beginning of the third week of pregnancy.

In the first week it is impossible for you to feel symptoms .

The pregnancy tests

In these weeks it is still too early to take a pregnancy test. However, it is never too early to find out how it works and when it should be used. For this reason we dedicate this short paragraph to introduce the pregnancy test. The pregnancy test works based on the detection of human chorionic gonadotropin – also known as hCG or human Chorionic onadotropin – a hormone produced by a cellular tissue called the trophoblast . The trophoblast is a superficial single-cell layer of the blastocyst which gives rise to the placenta and other embryonic annexes, but does not participate in the constitution of the embryo itself.  

hCG can be found:

  • in the blood , even before the first missed menstrual period, already seven to ten days after the presumed conception. This is the most accurate verification.  
  • in the urine , with the classic domestic device,  on the first day of delayed menstruation, which corresponds to about 14 days after conception. There are some highly sensitive urine tests that can be used as early as 4-5 days before the date you would expect to have your period.

The changes in the woman’s body

The female reproductive system. Click to enlarge.

Month after month, during menstruation the adult woman’s body does the preparatory work for becoming pregnant.

During this time various complex hormonal changes take place in order to get your body ready for an eventual conception in the next two weeks.

At each menstrual cycle, in the ovaries, from a group of small follicles that contain immature oocytes, a dominant follicle is selected which matures until it releases, through the so-called ovulation, the mature oocyte which can be fertilized by the sperm. 

The embryo in the first two weeks of pregnancy

Fertilization of the oocyte and formation of the embryo take place towards the end of the second week of pregnancy. A few minutes after the meeting between the male sperm and the female oocyte and conception, a new cell is formed, called a zygote, which contains 46 chromosomes, 23 of which are inherited from the paternal sperm and 23 of which from the maternal egg cell.

After a few hours, the zygote begins to move from the tubes to the uterus. During this process of approach to the uterus, the zygote begins a series of divisions, to form a cluster of cells in the shape of a blackberry, called blastocyst.

Subsequently the blastocyst will implant itself in the mother’s uterus, effectively concluding the phase of conception.

First weeks of pregnancy: what to do

In case you are pregnant, or think you might be,

  • consult with your doctor / obstetrician / gynecologist to evaluate the existence of any risk of genetic diseases of the fetus;
  • educate yourself about the changes that you should make in your everyday life so that you can enjoy the nine months to the fullest. Some changes you need to introduce into your life right now are: quitting smoking, stopping alcohol, reducing caffeine intake , avoiding drugs, not eating raw foods;
  • ask your obstetrician/gynecologist about environmental factors that could affect your pregnancy.
  • ask yourself what is the daily dose of folic acid that it is preferable for you to take in the coming weeks and months. Generally the recommended daily amount of folic acid  is 0.4-0.8 mg. Also consider that, in nature, folic acid is present in green leafy vegetables (lettuce and spinach, fruit and legumes) so try to make sure that one or more of these foods are present in your diet;
  • do regular physical activity: aim, in particular, at strengthening and lengthening the spine. This will allow you to carry your baby’s weight better in the following months.

Some curiosities regarding the period between the 1st and 2nd week

  • The genetics of the child is determined starting from fertilization. Since then, the gender, hair color, eye color, and hundreds of other genetic traits of your future child have been fixed.
  • It is the sperm that determines the sex of the baby. In fact, each sperm contains X and Y chromosomes while the egg contains only XX chromosomes.
  • The hCG hormone (human Chorionic Gonadotropin) is produced at the beginning of the second week after conception by the embryonic tissue only when a pregnancy is in progress; and home pregnancy tests are colored by the effect of this hormone.

Katherine Johnson, M.D., is a board-certified obstetrician-gynecologist with clinical expertise in general obstetrics and gynecology, family planning, women’s health, and gynecology.

She is affiliated with the Obstetrics and Gynecology division at an undisclosed healthcare institution and the online platform,

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