Pregnancy begins with the fertilization of sperm and an egg (ovum) which usually occurs in 2 weeks following the first day of the last menstrual period.
After the fusion of gametes, fertilized egg or zygote at 16 cell stage enters the uterine cavity for implantation. Thereafter, fetal development starts.
You may not know that you are pregnant in the first few weeks until you miss your period. You may not experience any symptoms for the first six weeks. But you can confirm your pregnancy by checking with the pregnancy kit when you missed your period.
Every woman wishes to know how her baby is developing in the uterus. So let’s know about the development of the fetus from contraception to the term .
Important events following fertilization
Fertilization of sperm and ovum occurs on day 15th following the last menstrual period.
Fertilized ovum (zygote) reaches 2 cell stage (blastomeres) through cell division.
4 cell stage of zygote by continuous cell division.
16 cell stage fertilized egg enters the uterine cavity.
Formation of blastocyst occurs.
The process of implantation is completed by the 11th day. Implantation usually occurs in the endometrium of the anterior or posterior wall of the uterus near the fundus which corresponds to the 26th day of a regular menstrual cycle.
Formation of primary, secondary and tertiary villi is completed.
The fetal heart is formed with the establishment of fetal placental circulation. The heart starts to beat.
The embryonic period usually begins at the 3rd week following fertilization and extends up to 8 weeks of gestation.
- The embryo is about 4 to 5 mm in length.
- Buds of arms and legs begin to form.
- Foundations for the nervous system, genitourinary system, skin, bones and lungs are formed.
- The head is extremely prominent representing almost one-third of the entire embryo.
- Rudiments of eyes, ears and nose appear.
5 to 8 weeks
- The embryo is about 2.7 to 3.1 cm in length and weighs 2-4 gm.
- Major organs are formed but not fully developed.
- The embryo begins to assume human form. and markedly bent.
- Sex differentiation begins.
- Bones begin to ossify.
- Heartbeat can be detected.
- Purposeful limb movements begin.
- Eyelids are formed but sealed shut; fingers and toes are developed.
The fetal period begins after the 8th week following conception and ends in delivery.
9 to 12 weeks
- The foetus average length is 5-8.7 cm and its weight is 45 gm.
- The placenta is formed completely and the exchange of nutrients and waste products takes place between mother and foetus.
- Fetal circulation is complete and you can hear your baby’s heartbeat by using a fetal doppler instrument.
- Fingers and toes have soft nails.
- Buds for all the temporary baby teeth are present.
- External genitalia shows definite characteristics.
- All the body organs are formed and continue to mature.
- Rudimentary kidneys secrete urine into the bladder which escapes later into the amniotic fluid.
13 to 16 weeks
- The foetus is 9.4-18.6 cm in length and its weight is 97 to 200 gm.
- Heartbeat is present.
- Lanugo (fine hair) develops.
- The Head is erect.
- Nasal septum and palate close.
- Fingerprints are set.
- Blood vessels are visible under the transparent fetal skin.
- Limb movements are present.
- In the female foetus, the ovaries are formed and contain primordial follicles that have oogonia.
17 to 20 weeks
- The foetus is 20.4-25.7 cm in length and weighs 260- 460 grams.
- Lanugo covers the entire body.
- Fetal movements can be felt by the woman.
- Eyebrows and scalp hair are present.
- By auscultation, heart sound can be heard.
- Vernix caseosa covers the skin.
- The uterus is developed and canalization of the vagina has begun in the female foetus by the 18th week and the testes are at the inguinal ring in the male foetus at about the 20th week.
21 to 25 weeks
- The foetus is about 27.4-33.7 cm in length and weighs 495-910 gms.
- Rapid eye movement begins
- Skin appears wrinkled and pink and red.
- Eyebrows and fingernails develop.
- Sleeps and wakes at regular intervals.
- Becomes more active and moves from side to side.
- The baby grows rapidly and sustained weight gain occurs.
26 to 29 weeks
- The foetus is 35.1-39.3 cm in length and weighs about 910 gms to 1.5 kgs.
- The Baby’s skin is red.
- Rhythmic breathing movements occur.
- The foetus often survives if born prematurely.
- The central nervous system has matured to the stage where it can direct rhythmic breathing movements and control body temperature.
- In the male foetus, scrotum descent may begin as early as the 28th week.
30 to 34 weeks
- The foetus is 40.5 – 45.3 cm in length and weighs 1.7 to 2.5 kg.
- The baby can open and close the eyes and suck the thumb.
- The baby responds to light and sound.
- Steady weight gain occurs
- Vigorous fetal movements occur.
- Toenails become visible.
35 to 37 weeks
- The foetus average length is 46.3 to 48.3 cm and weighs about 2.7 to 3.4 kg.
- Body and face have less wrinkled appearance because of fat deposit.
- Lanugo disappears.
- Nails reach fingertip.
- Amniotic fluid starts to decrease.
- The baby is too big to move around much but can kick strongly.
- The bones of the head are soft and flexible to make it easier for the baby passes through the birth canal.
- The nervous system is sufficiently matured.
- The foetus has a firm grasp.
38 to 40 weeks
- The foetus average length is 49.3 to 51 cm and weighs about 3.4 – 3.7 kg.
- The baby is at full term.
- Skin is smooth.
- The chest is prominent and the lungs are matured.
- Testes descent into scrotum.
- The fetus is engaged into the pelvis and ready for birth.
If you found any deviations from the normal growth and development of the foetus then you need to consult your health care provider.
In the above post, we have discussed the normal growth and development of the foetus week by week from conception to term. Any deviation from the normal development should be reported to the obstetrician.