Fetal development in the womb during pregnancy

Congratulations on becoming pregnant! Prenatal development is the process in which an embryo and later fetus develops during gestation. Each month a group of eggs (called oocytes) is recruited from the ovary for ovulation (release of the egg). The eggs develop in small fluid-filled cysts called follicles. You’re pregnant. We are sure you are curious about how your pregnancy will progress, and how your baby will develop week to week over the next few months.

Prenatal development starts with fertilization the first stage in embryogenesis which continues in fetal development until birth. Normally, one follicle in the group is selected to complete maturation. Congratulations! In this slideshow we will take a look inside the womb to see how a baby develops through the first, second, and third trimesters. In human pregnancy, prenatal development, also known as antenatal development, is the development of the embryo following fertilization, and continued as fetal development.

First 3 months

This dominant follicle suppresses all the other follicles in the group, which stop growing and degenerate. You’ll undoubtedly spend the months ahead wondering how your baby is growing and developing. Step one of conception is when the sperm penetrates the egg to complete the genetic make-up of a human fetus. By the end of the tenth week of gestational age the embryo has acquired its basic form and is referred to as a fetus. The mature follicle opens and releases the egg from the ovary (ovulation). Ovulation generally occurs about two weeks before a woman’s next menstrual period begins.

What does your baby look like? At this moment (conception), the sex and genetic make-up of the fetus begins. The next period is that of fetal development where many organs become fully developed. After ovulation, the ruptured follicle develops into a structure called the corpus luteum, which secretes progesterone and estrogen. How big is he or she?

fetus developing inside the womb

About three days later, the fertilized egg cell divides rapidly and then passes through the Fallopian tube into the uterus, where it attaches to the uterine wall. This fetal period is described both topically (by organ) and chronologically (by time) with major occurrences being listed by gestational age. The progesterone helps prepare the endometrium (lining of the uterus) for the embryo to implant. When will you feel the first kick? The attachment site provides nourishment to the rapidly developing fetus and becomes the placenta.

In other animals the very early stages of embryogenesis are the same as those in humans. On average, fertilization occurs about two weeks after your last menstrual period. Fetal development typically follows a predictable course. After 4 weeks, the basic structures of the fetus have begun to develop into separate areas that will form the head, chest, abdomen, and the organs that are contained within them. In later stages, development across all taxa of animals and the length of gestation vary.

When the sperm penetrates the egg, changes occur in the protein coating around it to prevent other sperm from entering. Find out what happens during the first trimester by checking out this weekly calendar of events. Small buds on the surface will become arms and legs. The perinatal period (from Greek peri, “about, around” and Latin nasci “to be born”) is “around the time of birth”. At the moment of fertilization, your baby’s genetic make-up is complete, including its sex.

Month 4 to month 6

Keep in mind that measurements are approximate. A home pregnancy test should be positive at this stage of development (most tests claim positive results one week after a missed period). At 8 weeks, the fetus is about one-half an inch long (1.1cm). Facial features such as developing ears, eyelids, and nose tip are present. In developed countries and at facilities where expert neonatal care is available, it is considered from 22 completed weeks (154 days) of gestation (the time when birth weight is normally 500 g) to 7 completed days after birth. If a Y sperm fertilizes the egg, your baby will be a boy; if an X sperm fertilizes the egg, your baby will be a girl. It might seem strange, but you’re not actually pregnant the first week or two of the time allotted to your pregnancy.

The limb buds are now clearly arms and legs, while the fingers and toes are still developing. In many developing countries, the starting point of this period is considered 28 completed weeks of gestation (or weight more than 1000 g). In ICD-10, a medical classification list by the WHO, there is a particular chapter relating to certain conditions originating in the perinatal period. Human chorionic gonadotrophin (hCG) is a hormone present in your blood from the time of conception. Yes, you read that correctly! At 12 weeks, the fetus has grown to about 2 inches (4.4cm) in length and may begin to move by itself.


The antepartum period (from Latin ante “before” and parere “to give birth”) is literally equivalent to prenatal (from Latin pre- “before” and nasci “to be born”). Practically, however, antepartum usually refers to the period between the 24th/26th week of gestational age until birth, for example in antepartum hemorrhage. It is produced by cells that form the placenta and is the hormone detected in a pregnancy test. Conception typically occurs about two weeks after your last period begins. The fingers and toes are discernible and the fetal heartbeat may be audible by Doppler ultrasound. When semen is released into the vagina, the spermatozoa travel through the cervix and body of the uterus and into the Fallopian tubes.

However, it usually takes three to four weeks from the first day of your last period for the hCG to increase enough to be detected by pregnancy tests. To calculate your due date, your health care provider will count ahead 40 weeks from the start of your last period. The developing sex organs may be identified by ultrasound techniques. Fertilization of the egg cell (ovum), usually takes place in one of the Fallopian tubes. Within 24 hours after fertilization, the egg begins dividing rapidly into many cells.

This means your period is counted as part of your pregnancy — even though you weren’t pregnant at the time. At 16 weeks, the fetus is about 4 and one-half inches long and resembles an infant; the eyes blink, the heartbeat is easier to locate, facial features (nose, mouth, chin and ears) are distinct, and the fingers and toes are clearly developed; the skin on the fingers and toes even have distinct patterns (fingerprints!).