The zygote is the fertilized egg, which is formed during conception. It is not a stage of prenatal development, but rather the starting point.

The heart is the first organ to develop during the embryonic stage, beginning around the third week of gestation.

Implantation is the process by which the embryo attaches to the lining of the uterus, typically occurring around 6-10 days after fertilization.

The neonatal period is the first 28 days after birth, and is not considered a critical period of prenatal development.

The amniotic sac is a fluid-filled membrane that surrounds the embryo and fetus, providing a protective environment.

Folic acid deficiency, maternal smoking, and excessive alcohol consumption during pregnancy can all increase the risk of neural tube defects in the fetus.

Placental transfer is the process by which oxygen and nutrients from the mother's blood are transferred to the fetus through the placenta.

A blood test is not a common prenatal screening test. Ultrasound, amniocentesis, and chorionic villus sampling are all common prenatal screening tests.

Progesterone is the hormone that is produced by the placenta and helps to maintain pregnancy.

Constipation is not a common symptom of pregnancy. Nausea, vomiting, and fatigue are all common symptoms of pregnancy.

The average length of a human pregnancy is 40 weeks, or 280 days, from the first day of the woman's last menstrual period (LMP).

Maternal age under 18 is not a risk factor for preterm birth. Multiple pregnancy, maternal age over 35, and previous preterm birth are all risk factors for preterm birth.

Low birth weight is the condition in which the fetus is born with a birth weight below 2,500 grams (5 pounds, 8 ounces).

Preeclampsia is not a common cause of fetal death. Chromosomal abnormalities, placental abruption, and maternal infection are all common causes of fetal death.

Birth defect is the condition in which the fetus is born with a structural or functional abnormality.