We continue with our special “Pregnancy Week by Week”: after week 1 y the 2, we come to fertilization. One of the most wonderful and fascinating processes in nature is fertilization.Beyond the classic image of the union between the egg and the sperm, this process involves a series of amazing steps that were rarely explained to us in detail in class and that at "Mothers Today" we want to reveal to you calmly.
We're in that perfect chemical dance where two sex cells, or gametes, exchange their genetic material, but not before engaging in the most demanding race: millions of sperm compete so that only one can reach the egg. What happens next holds one of the greatest mysteries and challenges for science. the moment when two tiny cells trigger a cascade of transformations that will give rise to “life”We'll explain it to you in detail, step by step, and also what happens in your body during this third week of pregnancy.
Fertilization step by step
Did you know that an egg cell is one of the largest cells in the human body? Although still microscopic, its size is considerable compared to that of a sperm. Each ejaculation can contain between between 120 and 600 million spermBut only a small part will manage to get close to the egg, and barely a few hundred will reach the area where fertilization can occur.
From a medical point of view, the third week of pregnancy is usually considered the first week of embryonic developmentHowever, by consensus, pregnancy is counted from the first day of the last menstrual period (gestational age). Therefore, This week marks two key milestones: fertilization and the beginning of embryo development.
In the final part of the second week or the beginning of the third, the ovulationThat is, the release of the oocyte from the ovary into the fallopian tube. These days constitute the fertile window: if there is unprotected sex, the sperm will already be waiting in the fallopian tubes when the egg arrives and conception will occur.
A long journey begins
This adventure begins when semen is deposited in the vagina. This environment is acidic and unfavorable For sperm survival, they must move quickly to more alkaline areas. Within hours, the fittest ascend the cervix, pass through the uterus and reach the fallopian tubes.
Once in the fallopian tubes, sperm can survive for approximately between 48 and 72 hoursThis explains why fertilization can occur up to two or three days after sexual intercourse. During this time, the vast majority of the millions of sperm ejaculated are lost, leaving only a few hundred capable of reaching the egg.
On this journey, sperm travel through the cervical mucus, the uterus, and the fallopian tube, aided by muscle micro-contractions of these structures and by the movement of their tails. The inside of the fallopian tubes is lined with millions of cilia, tiny "hairs" that move in a coordinated fashion and facilitate the movement of both the sperm and, later, the embryo.
During this journey, the sperm undergo essential changes in their membrane, a process called capacitation: they partially lose the coating of their heads, which allows a series of chemical reactions essential to penetrate the layers that protect the egg to be activated.
Penetration of the radiated crown
To better understand the stages of fertilization It's helpful to imagine the ovum as a kind of capsule protected by several layers. The outermost layer is the radiate corona, formed by follicular cells that surround the ovum and provide it with nutrients and proteins.
When sperm reach this area, they must make their way through. through the cells of the corona radiataThey achieve this thanks to specific enzymes released from the sperm head, which allow them to "tunnel" between cells. It's a true microscopic battle where only the most capable advance.
Arrival in the zona pellucida
After passing through the corona radiata, the sperm encounter a second barrier: the zona pellucidaIt is a glycoprotein coating that surrounds the ovum and performs a crucial function: allow the entry of a single sperm and to prevent multiple fertilization.
To get through it, some sperm initiate the so-called acrosomal reactionIts acrosome (the "capsule" located on the head) releases enzymes that penetrate the zona pellucida. Not all sperm succeed, but this combined effort makes it easier for one of them to eventually get close enough.
At that moment a very particular structure appears: the acrosomic filamenta projection that helps the sperm to make contact with the egg's membrane. This contact is the step prior to final fusion.
The fusion
Although only a few sperm have reached the zona pellucida, that “battle” for survival and enzymatic reactions It will allow luck to favor only one: the one that manages to make contact with the oocyte's plasma membrane thanks to the acrosomal filament. When this happens, a cascade of fascinating changes is triggered.
First, the membranes of the sperm and the egg begin to fuseThis process involves ionic changes on the surface of the egg that block the entry of other sperm. This is known as blockage of polyspermy, essential to ensure a correct number of chromosomes.
Then comes the so-called fertilization cone, a structure in the egg membrane that facilitates the entry into the oocyte cytoplasm of the sperm head, midpiece and, in part, tail.
Once inside, the sperm nucleus moves towards the egg nucleus. Fertilization is completed when both nuclei fuse.Each sex cell contributes 23 chromosomes, so that when they unite, the 46 chromosomes of the human species are reached. At that moment, the chromosome is formed. zygote, the first cell of a new individual.
Although half of the genetic material comes from each parent, the specific combination of those chromosomes and genes is uniqueA unique individual has been created, with a genetic sex already determined by the sex chromosomes: if the sperm contributed an X chromosome, it will be XX (girl); if it contributed a Y chromosome, it will be XY (boy).
From here, this zygote begins a intense series of cell divisions and changes that, after many months of growth, will give rise to a new human being.
The following video It will help you visually understand the wonder of fertilization:
https://www.youtube.com/watch?v=–tXsEAUKqo
Beginning of embryonic development: from zygote to blastocyst
For guidance purposesThe days following fertilization can be described as follows:
- Day 0: union of the ovum and the sperm and formation of the zygote.
- Day 1At 16-20 hours post-fertilization, the embryo is still a single cell with the genetic material fused.
- Day 2The first segmentations give rise to an embryo of about 4 cells.
- Day 3: the embryo reaches approximately 8 cells.
- Day 4The embryo already has at least 16 cells and is called morulabecause of its resemblance to a small blackberry.
- Day 5The morula hollows out, fills with liquid, and is then called blastocyst, with about 200 specialized cells.
- Day 6The blastocyst expands, continues to increase its number of cells, and prepares for implantation.
Within the blastocyst, two well-defined cell groups can be distinguished. The first, located in the inner part, is called inner cell mass and will give rise to the embryo itself. The second, on the outside, forms the trophoblastwhich will be the origin of the placenta and the structures that will nourish and protect the baby throughout the pregnancy.
At the end of the third week of pregnancy, the embryo is at the blastocyst stage it reaches the uterus and begins to look for a suitable place in the endometrium to implant.
Embryo's journey through the fallopian tube and implantation
After fertilization, the zygote and, subsequently, the embryo in its various stages travel down the fallopian tube towards the uterus. This journey is possible thanks to the gentle contractions of the fallopian tube muscles and to the coordinated movement of the cilia that cover its inner surface.
La progesteroneThe hormone secreted by the ovary after ovulation plays a key role in this process. This hormone, known as the "pregnancy hormone," helps relax the sphincter that connects the fallopian tube and uterus, facilitating the passage of the embryo. It also transforms the endometrium, making it thicker, more vascularized, and richer in nutrients, increasing the fertility of the uterus. endometrial receptivity and facilitating implementation.
When the blastocyst reaches the uterus, the process begins implantationThis is a process by which the embryo attaches to and embeds itself in the uterine wall. This attachment is essential, as it establishes the connection that allows the exchange of oxygen, nutrients, and waste products between the mother and the embryo. Over time, this implantation site will become the placenta.
During implantation, a slight bleedingThis is known as implantation bleeding, which many women mistake for a lighter period. It is usually short-lived, light in amount, and lighter in color than a regular menstrual flow.
What changes occur in the mother during week 3
Although the activity inside you is intense, In the third week of pregnancy, most women still don't notice major changes.In fact, many do not yet suspect they are pregnant, as there is still approximately one week until the date they expected their period.
In women undergoing fertility treatments such as artificial insemination (AI) or in vitro fertilization (IVF), this phase is known as two weeks' waitIn these cases, some discomfort may be more related to hormonal medication than to the pregnancy itself.
Nevertheless, the hormonal changes These changes that begin to occur can lead to symptoms such as:
- Breast tenderness or discomfort.
- Increased perception of smells and changes in taste.
- Salivation slightly more abundant.
- Mood swingsirritability or heightened emotionality.
- Tiredness and drowsiness more than usual.
- Headache slight or heavy feeling.
- Increased urge to urinate due to hormonal changes.
- White, thick vaginal dischargewhich is usually physiological.
It is important to remember that It's also completely normal not to feel different at this stage. Every body reacts uniquely, and some women don't experience any symptoms until weeks later.
Is it possible to take a pregnancy test in week 3?
In the third week of pregnancy, especially before a missed period, A urine test can produce false negatives. The beta-hCG hormone (beta fraction of human chorionic gonadotropin), which is what pregnancy tests detect, only begins to be produced significantly when the embryo has implanted.
For this reason, the most reliable thing is wait at least until you have a missed period to take a test. Some highly sensitive tests can detect pregnancy a few days earlier, but the margin of error is greater. In women undergoing assisted reproductive treatments, hormonal medication can even cause false positives if the test is performed too early.
If a more accurate result is desired at such early dates, a beta-hCG blood test It can detect low levels of the hormone somewhat earlier than urine tests, although it is still advisable to respect the times indicated by healthcare personnel.
Recommended care during the third week of pregnancy
Even though you don't have confirmation yet, the mother's habits and lifestyle From the very beginning of pregnancy, these factors influence the baby's development. Therefore, if you are trying to conceive or suspect you might be pregnant, it's advisable to start taking care of yourself now.
Food and supplements
El folic acid It's one of the key nutrients at the start of pregnancy, as it helps prevent neural tube defects (the structures that develop into the brain and spinal cord). It's generally recommended to start taking it even before conception and continue throughout the first trimester. You can find it at leafy green vegetables and legumesalthough it is often prescribed as a supplement as well.
In addition, it may be advisable to increase consumption of iron through diet or supplements, as assessed by your healthcare professional. This mineral is essential for hemoglobin formation and for preventing anemia. red meat in moderationLegumes and some vegetables are good sources, and combining them with foods rich in Vitamin C (strawberries, citrus fruits, tomatoes) improves its absorption.
Another important mineral is CalciumIt is necessary for the baby's bone and muscle development and to protect your own bone and cardiovascular health. Dairy products, fortified drinks, nuts, and leafy green vegetables provide good amounts.
From the moment you suspect you might be pregnant, it's advisable avoid certain foodsAvoid raw meat and fish, fish with high mercury content, undercooked eggs, unpasteurized cheeses, and raw sausages if you are not immune to toxoplasmosis. It is also important to wash fruits and vegetables thoroughly to reduce the risk of infections such as listeriosis.
At this stage, it is also advisable limit caffeine and completely suppress the alcohol, as it can affect embryonic development even in very early stages.
Physical exercise and rest
Staying moderately active helps improve circulation, control weight, and regulate mood. If you used to do intense exercise, now is a good time to... adapt it to gentler activities, such as walking, prenatal yoga, adapted pilates or swimming, unless your doctor advises otherwise.
Equally important is the adequate restSleeping between 7 and 9 hours, establishing sleep routines, and finding moments of relaxation during the day will help manage the typical tiredness of these first few weeks.
No tobacco from the start
El tobacco It is associated with pregnancy complications such as miscarriage, premature rupture of membranes, placental abruption, intrauterine growth restriction, and low birth weight. It has also been observed that children of mothers who smoke are more likely to have these conditions. respiratory infections, bronchiolitis, otitis and asthma in the childhood.
Nicotine has a vasoconstrictor effect
Smoking during pregnancy increases the risk of sudden infant death syndromeWe know that nicotine, carbon monoxide, and other mutagenic and carcinogenic substances cross the placenta and reach the baby; traces have been found in the blood, urine, skin, and even teeth of children of mothers who smoke. nicotine has an effect vasoconstrictor in the placenta and fetal circulation, reducing the supply of oxygen and nutrients to the fetus.
(Note: As a feminist organization, we are committed to the inclusive languageTherefore, and to avoid making the reading tedious, in some cases we will speak of "son" or "little brother" and in others of "daughter" or "little sister"; sometimes we will refer to the "baby" and in other cases to the human "creature".)
Your baby and your body in these first three weeks
Your baby has been developing inside you since the moment of conception. Around the middle of your menstrual cycle, a woman releases an egg from one of her ovaries into the fallopian tube. For about 24 hours, there is a chance of conception if a sperm fertilizes it.
From that moment on, the genetic sex of the creature The sex chromosomes determine the fertilized egg, and intense cell division begins. While changes are occurring inside the fertilized egg, it travels to the uterus, where the sperm awaits. endometrium, the tissue that forms each month and that we expel during menstruation when there is no pregnancy.
La implantationImplantation, the process by which the embryo attaches to the uterine wall, begins toward the end of the third week and is usually completed during the fourth. During this period, the uterine lining also begins to form. amniotic fluid sac and the appears umbilical cordwhich will be the permanent physical link between your body and your baby throughout the pregnancy.
During these first three weeks, your body may begin to notice physical changes such as nausea, aversion to certain smells, drowsiness, or emotional changesAlthough you might not notice anything special. Everything is within the realm of normal, because EVERY WOMAN IS UNIQUE, EVERY PREGNANCY IS UNIQUE, AND EVERY CREATURE IS UNIQUE.
If you are taking any medication or have a chronic illness, it is essential that Consult with your midwife or doctor when you suspect you are pregnant. Even before conception, a supplement may be recommended. folic acid and, in some cases, other nutrients such as iodine, always under professional supervision.
Week 3 of pregnancy is a quiet time on the outside, but incredibly active on the inside. During these days, much of your baby's future development is determined, their genetic material is organized, the journey to the uterus begins, and implantation starts, while your body begins to adapt hormonally. Simple gestures Taking care of your diet, avoiding toxins like tobacco and alcohol, getting enough rest, and consulting your healthcare team with any questions can have a very positive influence on your child's health now and in the future.