The first 13 weeks are known as the first trimester. Week 14 to 27 is the second trimester. And week 28 to 40 is called the third trimester. Each trimester is marked by a steady progression of changes that will prepare both Mom and baby for birth. These, too, help make baby ready to live and survive outside her mother’s womb.
How your baby develops
First Trimester of Pregnancy: Week 1 - 13
The first trimester is a time of tremendous development. There is only a window of time --- only 12-24 hours after ovulation when an egg can be fertilized by a single sperm. If no fertilization happens, the endometrial lining of the uterus flows out as menstruation when the woman has her next period.
Your baby is 2 millimeters or the size of a mustard seed*. Three distinct layers of cells start to form. The Ectoderm (outer layer), will eventually become the baby's skin, eyes, hair, nervous system, brain and enamel of teeth. The Mesoderm (middle layer) will develop into her skeleton, muscles and kidneys, tissues and blood system. The Endoderm (inner layer) will eventually become her internal organs.
Your baby is now officially called a fetus. Its fingers, toes and lips, eyelids and legs are becoming more clearly defined. Your baby's bones are beginning to form. So much of your baby's growth is concentrated on their brain this month, with about 100 new brain cells forming every minute. Your baby's kidneys are already in its proper place, but have not started filtering baby's blood just yet. Soon, these kidneys will produce urine, which will form a good part of the amniotic fluid in which baby is going to float around for the next 7 months.
Your baby is moving all the time, kicking and stretching, twisting and turning. But because she is still so small and your uterus is still at the top of your pelvis, you can’t still feel her moving. Baby's bone marrow is starting to produce white blood cells, the infection-fighting cells that help her stay well and healthy. Your baby's placenta is now doing its all-important work of filtering oxygen and nutrients to help her grow. This month, your baby may suck her thumb. Eyelids will fully cover her eyes so they are well-protected. Your baby's intestines start contracting and relaxing, in preparation for healthy digestion when she is born.
Second Trimester of Pregnancy: Week 13 - 28
The second trimester is when baby’s vital body organs mature and her nervous system develops. Weeks 13-18 are a major, critical period in your baby's brain development. Avoid any alcohol, tobacco or drugs now because these could have long-term effects on your child. All throughout this time, baby will grow in size and maturity. By middle of the second trimester, feel na feel na ni Mommy ang bawat sipa ni baby. Many mothers start to "bloom", feel more energetic and generally better than the first trimester. Nausea tends to settle and it is still too early for movement to be restricted.
Trimester of Pregnancy: Week 29 – 40
By the third trimester, baby has regular periods of rest and activity. Frequently changing her position, baby kicks and moves until she finds a comfortable pose in the uterus. Your baby's brain and nervous system are fully formed by now while her lungs continue to mature.
For a pregnant mother, this can seem like the longest of the three trimesters. It is often clear she is pregnant and her size a constant reminder of what the future holds. Even simple activity can become tiring, especially if work and caring for other children is a reality. It can't be hurried though. The baby still needs to be fully supported by its mother and if it is born too early, will require special care.
Co-written with Jennifer Navaja, RND.
Nelson textbook of Pediatrics, 20th edition. Ed, Kliegman, RM, Elsivier, Philadelphia, PA; 2011.
Pediatric Nutrition, 7th edition. Ed, Kleinman, RE, American Academy of Pediatrics, Elk Grove Village, IL; 2014.
Sadler, T W, (Thomas W.); Langman, Jan.Langman's Medical Embryology.11th ed., Philadelphia : Wolters Kluwer Lippincott Williams & Wilkins, 2010.