The term prenatal development refers to the development of your baby in the womb, from a single cell to a newborn. It is different from the three trimesters of pregnancy, which divide the time period. Pregnancy is described as lasting nine months or 40 weeks. There are three key developmental stages: germinal, embryonic, and foetal, and all are exciting and sometimes nerve-racking for expectant parents. However, knowledge is power, and understanding each stage well can build a strong foundation for further antenatal education, as well as allow you to ask informed questions at your antenatal appointments.

The Germinal Stage of Prenatal Development

The germinal stage is everything that happens from conception to the implantation of the fertilised egg in the lining of the womb. Healthy conception happens in the fallopian tube, with a sperm meeting an egg to form a zygote. Your baby’s gender is determined at this point. There’s nothing that happens in the womb or that the mother does during the pregnancy to affect any of these phenomena. Everything at this stage is microscopic, and it is the shortest developmental stage, lasting about two weeks. After conception, a process of cell division called mitosis happens rapidly as the zygote travels through the fallopian tube to the uterus, separating into an outer layer, which will grow to form your baby’s placenta, and the internal sphere containing cells that will later form its organs, brain, spine, and amniotic sac. This zygote of differentiated cells is called a blastocyst. It embeds itself in the uterine lining, typically around the top, which can cause some light bleeding, though it is also normal not to bleed. It begins producing human chorionic gonadotropin, which stops your period, supports your pregnancy, and can cause a pregnancy test to show a positive result. The placenta starts developing soon after implantation.

The Embryonic Stage of Prenatal Development

The embryonic stage lasts from two to eight weeks, during which time the blastocyst takes on more human characteristics and is known as an embryo. The embryo grows rapidly during this time, with lots more cell differentiation. From weeks three to eight, three layers start to form your baby’s organs, while others form their neural tube and limbs, as well as the yolk sac that nourishes them before the placenta takes over (usually around eight to 12 weeks). The amnion forms a sac filled with amniotic fluid that forms around the foetus and allows it to move. The chorion develops into the placenta, which acts as your baby’s lifeline, allowing for the exchange of blood, fluids, and gases between you. By the beginning of the foetal stage, all your baby’s major organs will be formed, though they will continue to grow and mature throughout the pregnancy. Folic acid’s crucial role in healthy neural tube development during this stage is one of the reasons pregnant women are so strongly advised to supplement daily, ideally starting at least three months before conception.

The Foetal Stage of Prenatal Development

The foetal stage lasts from week nine until birth, though you will most likely refer to “the baby” rather than “the foetus” in conversation, especially once you start to feel him or her moving around (usually from around 18 weeks, though for first-time mums it’s normal for it to start a little later). During this stage, the foetus grows and develops rapidly. Their height and weight will be measured at antenatal appointments, and extremes recorded in either direction (i.e., your baby being small or large for gestational age) will cause your pregnancy to be classified as high risk and more closely monitored. Facial features are defined between weeks sixteen and twenty weeks. Your baby’s gender can be identified by the sonographer at your anomaly scan (around 20 weeks), though some providers may reveal this earlier. Your pregnancy will be termed “viable” after 24 weeks, which means if they were born from this week onwards they will likely survive, though they are more likely to survive if born after 28 weeks.

For more advice on taking care of yourself in pregnancy, labour, and birth, check out Unii’s Antenatal Classes, with prenatal video episodes taught by senior NHS midwives, an award-winning obstetrician and gynaecologist, and six other leading healthcare experts. Only at unii.com.

Antenatal Class Questions: What are the stages of prenatal development?