Summary
Highlights
The human gestation period spans approximately 280 days or 40 weeks, starting from the last menstrual cycle before conception and ending with birth. This period is divided into three trimesters, each with significant developmental events, followed by the birth process, also known as parturition.
The first trimester begins with fertilization, where sperm meets the egg to form a zygote. This zygote undergoes cleavage, blastulation (forming a blastocyst), and then implants in the uterine lining. Implantation leads to placenta formation and gastrulation, establishing the three germ layers. Neurulation, the formation of the nervous system, also occurs, alongside the development of the heart, lungs, digestive system, limbs, and gonads, determining the fetus's sex by the end of this trimester (up to week 13).
From week 14 to week 27, the second trimester sees accelerated heart development, with the fetal heart rate increasing to about 100 beats per minute. The fetus begins to move, and the mother can feel these movements. Facial features, including eyes, ears, and nose, become more defined, and the fetus starts to resemble a human being.
The third trimester, starting from week 28 until birth, is characterized by very rapid fetal growth. All organs undergo final differentiation, with the cerebrum developing fully and forming convolutions. Reflexes like grasping and suckling become apparent, and the fetus may begin sucking its thumb. Hair growth may also occur.
The birth process, or parturition, is divided into three stages. Stage one, lasting about 12 hours, is labor. Hormones stimulate uterine contractions, positioning the fetus's head towards the cervix. The cervix dilates, relaxes, and flattens out, allowing passage. Towards the end of this stage, the amniotic sac ruptures, releasing amniotic fluid, which signals the impending delivery.
Stage two, lasting 20 minutes to an hour, is the actual birthing process. The mother combines abdominal muscle contractions with uterine muscle contractions to push the fetus through the cervix and vaginal cavity, expelling it into the outside environment. The newborn is then referred to as a neonate.
Stage three, known as afterbirth, lasts about 10 to 15 minutes. The uterus continues to contract to loosen and expel the placental membrane, which is still attached to its wall. After the umbilical cord is cut, the uterine lining begins to regenerate, and the menstrual cycle eventually resumes, as it is suspended during pregnancy.