Strange as it may seem, your doctor or midwife will calculate your due date (and your baby's gestational age) by working back to the first day of your last menstrual period. Since it's often difficult to know exactly when you're ovulating, and consequently when sperm fertilised your egg, medical experts often use your last menstrual period as the starting point for the next nine months. This means the first 'official' week of your pregnancy is actually the week you started your last period. As a result, your baby's real age is two weeks behind his gestational age.
An average full term pregnancy consists of a nine month gestation period - approximately 280 days. The first three months are called the First Trimester. During the First Trimester, the baby develops all of its internal organs, the sense organs and the arms and legs. The Second Trimester is the fourth through the sixth months; the baby continues to develop during this time. The Third Trimester is the seventh month until birth; the baby finishes developing and gains the bulk of what will be the birth weight during this time.
Immediately upon conception, cellular development begins. These cells become implanted in the uterine walls of the mother around five to nine days after fertilization. At approximately fourteen days, the expectant mother will miss her first period. At around twenty-four days and up until the end of the Trimester, the baby will begin to develop a heartbeat, a nervous system, a skeletal system, muscles and blood will begin to flow through its vascular system. Also, the eyes, ears, nose, fingernails, eyelids (although they are fused shut) and limbs will start to develop. The baby will start moving around in the twelfth week, but the expectant mother will not be able to feel it yet. The baby inhales amniotic fluid and can suck its thumb now also. At the end of the First Trimester the baby is technically no longer an embryo, but is now called a fetus.
In the fourth month of the Second Trimester, the baby will have developed a strong heartbeat. The expectant mother should now be having physical signs of her pregnancy. Because the baby has a developed digestive system it will become more active because of the food, water and oxygen it is now receiving. This month the eyebrows and the genital organs have also developed (although the sex of the baby was determined at conception). http://www.essortment.com/development-stages-baby-conception-birth-50825.html During the first half of the menstrual cycle, hormones stimulate growth and development of a single follicle within one of the ovaries. At the same time, another hormone stimulates growth of the uterus lining in preparation for the fertilized egg.
When the egg cell matures, hormone levels surge and trigger the egg’s release. The exact timing of ovulation relates to a woman’s individual menstrual cycle (period). In an average 28-day cycle, ovulation occurs between days 14 and 16, day one being the first day of menstruation. The egg follicle bursts, releasing a fertile egg, which migrates to the fallopian tube. What’s left of the spent follicle helps produce a hormone, which prepares the endometrium or uterus lining for the implantation of the fertilized egg.
En route from the fallopian tube to the uterus, the egg may be fertilized. Sperm can live within the human body for 48-72 hours. But of the countless sperm that make it to this stage, only one may penetrate the egg’s protective membrane and fertilize the egg. During fertilization, the couple’s genes combine to create an embryo.
As the first few cells divide, the embryo continues its course toward the