The week at which embryos develop is around week 3 after fertilization occurs or week 5 of pregnancy, which is when the blastocyst implants and the first cells also differentiate and progress rapidly through week 8 to form major organs, and after it's then called a fetus.
By week 4, key structures like the heart, spinal cord and brain start forming and the heart often starts being around week 6 to 8, which then marks the transition of the embryo into the fetal stage where the refinement and growth continue.
Embryos develop a heartbeat very early and often around 22 days after conception, which is around the 6th week of pregnancy.
A simple tube for the heart develops and begins to pulse and pump blood, although it's not yet a fully formed 4 chambered heart just yet.
The early cardiac activity in an embryo is detectable by ultrasound though and shows a rhythmic contraction and the rate so increases significantly by around 110 bpm at 6 weeks to over 170 bpm by 9 weeks.
The first organ to develop in a fetus is the heart, which forms as a simple tube around the third week of gestation and begins to beat by week 4.
The heart developing in a fetus is crucial for pumping oxygen as well as nutrients throughout the rapidly growing embryo.
The early development of the cardiovascular system and heart also enables the transport that is necessary for all other organs to form and function.
The heart of the fetus starts forming just a couple of weeks after fertilization and by week 3 a primitive heart tube forms from specialized cells in the middle layer or mesoderm of the embryo.
A fetus develops in a woman's body through fertilization, implantation and also stages of rapid cell division and differentiation over 4 weeks, being housed in the mothers uterus, being nourished by the placenta and supported by maternal hormones, which progress from a single cell to a complex by with forming of organs as well as movement, and sensory functions, which ultimately results in birth.
During the early stages, once a woman has sex, the sperm meets an egg in the fallopian tube and forms a zygote, the zygote then travels to the uterus and divides into a ball of cells that are called a blastocyst.
The blastocyst then burrows into the uterine lining and triggers pregnancy hormone production, the blastocyst then becomes an embryo, with cells that specialize to form major organs like the brain, heart and spinal cord and external features.
The placenta then forms and connects the blood supply of the mother to the baby for nutrients and waste removal.
During the second trimester, the fetus starts moving and kicking and stretching, which the mother is able to feel at around 20 weeks.
The fetus can also then hear sounds and it's eyes become sensitive to light, fine hair also known as lanugo and protective skin cream called vernix develop and the digestive system works, lungs begin producing surfactant and bone marrow produces blood cells.
Then in the third trimester of pregnancy, significant fat is added to the baby for warmth after birth and proportions become more newborn like and lungs, brain and nervous system complete most of their development, bones harden, except the skull and the fetus often turns head down to prepare for birth.
And then around 38 weeks to 40 weeks, the baby is then mature enough to be born and survive out of the womb.
The role of the woman's body in pregnancy and fetus development is to provide a protective environment for the baby for 9 months, act as a vital lifeline, exchanging of oxygen, nutrients and waste and then to maintain the pregnancy the mothers estrogen and progesterone levels rise and support fetal growth.
A human baby forms in the mothers womb through fertilization of an egg by a sperm, creating a single cell that rapidly divides as it travels to the uterus, implants, and develops into an embryo (first 8 weeks) and then a fetus, with major organs and structures forming, nourished via the placenta and umbilical cord, until birth.
The human baby making process and pregnancy involves fertilization or conception, journey to the uterus, implantation, embryonic stage which is weeks 1 to 8 and the fetal stage which is week 9 to birth.
For the fertilization process, a sperm fertilizes an egg in the fallopian tube, which forms a single cell called a zygote.
Then the zygote divides into a ball of cells called a blastocyst as it moves to the uterus, which is a journey that takes around 1 week.
The blastocyst then burrows into the thickened lining of the uterus, and begins pregnancy and triggers hormone production.
During the embryonic stage at weeks 1 to 8, cells differentiate to form major systems like the brain, spinal cord, heart and organs and the placenta and umbilical cord form and provide oxygen and nutrients to the bay.
And by week 8, the embryo has recognizable human features and internal organs.
Then at the fetal stage which is from week 9 to birth, the embryo is now a fetus and growing rapidly, limbs, fingers, toes and facial features also become well defined and organs mature and the baby gains weight and develops reflexes and prepares for life outside of the womb.
The entire process of a baby growing and developing in a mothers womb, is from a single cell to a fully formed baby, is a complex series of cell multiplication, differentiation and structure formation, which is all supported by the mother's body and is an amazing process.