By six weeks from conception, other developmental milestones occur as circulation begins and a tube that will become the heart pulsates. At one month past fertilization, the baby has a mouth and a rudimentary tongue. Baby is now one-tenth of an inch long.
At eight weeks from conception ‑- the last stage of embryonic development ‑- the embryo has completed development of all internal and external structures and takes on a humanlike appearance. The tail is gone and the baby can move, but the movements are not yet able to be felt by the mother. Prenatal care should start at six to eight weeks, and by 10 weeks the fetal heart should be heard with an office Doppler device (unless the uterus is tipped back or the mother has a high body mass index). Baby is now about 1 1/4 inches long. The placenta and embryo are about the size of a golf ball.
By 12 weeks from conception, and weighing in at about an ounce, the baby is now called a fetus. Development is ongoing, and organs begin to take on functions that they will need in the outside world. The respiratory muscles assist in "breathing" in amniotic fluid and expelling it from the lungs. The heart is a real pump now, and circulation and the network of vessels are expanding. If exposed to air, the fetus could now make a sound. The genitalia may now be clearly identified as female or male, but on ultrasound these findings are still very subtle and can be misinterpreted. The placenta is functioning and hCG levels are declining, which means that nausea may be subsiding as well.
You have accomplished a great deal during the past three months! Further maturation and increase in cell number and size will continue until the end of the pregnancy. As baby puts on weight and brain development progresses, the quality of your nutrition takes on critical importance. Protein needs to increase to 60 grams per day with only a small increase required in calories (100 to 200 daily). Two protein servings at each meal should satisfy the needs; one could be a milk serving, and the other could be a serving of meat, cheese, beans, egg, nuts or peanut butter.
Fatigue should be less of a problem as the second trimester approaches. Exercise (30 to 45 minutes daily of brisk walking is a good option) is critical to keep the appetite under control, reduce the risk of excess weight gain and keep muscles toned to support the ligaments and bones and the maternal changes in weight. Your hard work will be rewarded with a healthier baby and less weight to lose in the postpartum period.