Interactive Tool: Critical Periods of Pregnancy

The first day of the last menstrual period is typically used to date a pregnancy.

The estimated date of conception is about two weeks after the LMP. This is the point in the cycle when the egg and sperm meet to create a zygote (the first cell that contains the instructions needed to grow into a baby).

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When a person is pregnant, a missed period happens about four weeks after the LMP. This is usually the earliest point at which a pregnancy can be confirmed.

The estimated due date is when the baby is expected to be born.

Most periods occur every 28 days and last anywhere from 2-7 days.

The all or none theory is that exposures in the first two weeks after conception will either have an all (miscarriage) or none (no risk) effect.

Once a body part has formed, birth defects are unlikely to occur. However, problems can sometimes happen after a body part has formed correctly. As the structures continue to grow and develop during the latter part of a pregnancy, medications, infections, genetics, health conditions, trauma, prematurity, and other issues can cause problems with how the body part works. Intellectual disability and hearing loss are examples of functional problems.

Before an organ starts to form, many complex changes are happening in the embryo to prepare for development. There is limited information about what is happening during this stage of pregnancy. This makes it difficult to know how early exposures could affect future development.

The neural tube is a structure that forms early in pregnancy. Once it closes, the neural tube develops into the central nervous system (brain and spinal cord), which is crucial for controlling our body’s functions, senses, and movements. Examples of birth defects that can occur when the neural tube does not form correctly during pregnancy are spina bifida (incomplete closure of part of the spine) and anencephaly (missing front part of the brain).

The lungs are a pair of organs in the chest responsible for helping with breathing. The lungs take in oxygen from the air you breathe and send it into your blood, which your body needs to work. The lungs also get rid of carbon dioxide, a waste gas your body doesn’t need, by pushing it out when you exhale. During pregnancy, the lungs are filled with fluid, but once the baby is born, they start breathing air. An example of a birth defect that can occur when the lungs do not form correctly during pregnancy is pulmonary sequestration (extra lung tissue).

The kidneys are two bean-shaped organs that remove waste products from the blood and produce urine. The kidneys break down drugs, help regulate blood pressure, balance electrolytes, and stimulate the production of red blood cells. An example of a birth defect that can occur when the kidneys do not form correctly during pregnancy is renal agenesis (missing kidney).

The heart is a strong, muscular organ located slightly to the left of the center of the chest. The heart works like a pump to send blood with oxygen throughout the body. The heart has four chambers: two upper chambers called atria and two lower chambers called ventricles. Blood enters the heart through the atria and is pumped out through the ventricles. The right side of the heart pumps blood to the lungs to pick up oxygen, and the left side pumps the oxygen-rich blood to the rest of the body. Examples of birth defects that can occur when the heart does not form correctly during pregnancy are ventricular septal defect (hole in the heart) and Ebstein’s anomaly (problem with the heart valve).

Eyes are the organs in your body that allow you to see. Light enters through the cornea and pupil, passes through the lens, and reaches the retina, which sends signals to your brain to create vision. An example of a birth defect that can occur when the eyes do not form correctly during pregnancy is anopthalmia/micropthalmia (small or missing eyes).

The arms are the two upper limbs of the body consisting of the upper arm, forearm, and hands. People use their arms to perform a variety of activities including lifting, pushing, pulling, throwing, climbing, and writing. An example of a birth defect that can occur when the arms do not form correctly during pregnancy is a limb reduction defect (missing or short arms).

The legs are the two lower limbs of the body consisting of the upper leg, knee, lower leg, ankle, and foot. The legs support the body’s weight and help with a range of movements including walking, jumping, sitting, and standing. An example of a birth defect that can occur when the legs do not form correctly during pregnancy is a limb reduction defect (missing or short legs).

The brain is a complex organ that controls many body functions. The brain receives, processes, and interprets information related to thoughts, decisions, memories, emotions, speech, touch, motor skills, vision, breathing, temperature, and hunger. An example of a birth defect that can occur when the brain does not form correctly during pregnancy is microcephaly (smaller than normal head and brain).

The ears are located on the sides of our head and are the parts of our body that let us hear sounds. Each ear has three main parts: the outer ear, the middle ear, and the inner ear. The outer ear is the part we can see, and it helps to catch sounds and send them into the ear canal. The middle ear has tiny bones that pass sound vibrations to the inner ear, where the sound is turned into signals that our brain can understand. Ears also help us keep our balance. An example of a birth defect that can occur when the ears do not form correctly during pregnancy is anotia/microtia (missing or malformed ears).

The abdominal wall includes the skin and muscles that cover the belly and help protect the internal organs in this stomach area. Examples of birth defects that can occur when the abdominal wall does not form correctly during pregnancy are gastroschisis (when the intestines stick out of a hole in the belly) and omphalocele (intestines develop outside of the body).

The axial skeleton is the part of the skeleton that forms the main framework of your body. It includes the bones of your head, neck, and trunk. This means it consists of the skull, the spine, and the rib cage. The axial skeleton supports and protects the brain, spinal cord, and organs in the chest. It also provides a place for muscles to attach, which helps you move and maintain your posture. An example of a birth defect that can occur when the axial skeleton does not form correctly during pregnancy is craniosynostosis (bones in the skull join together too early).

The lips are soft, movable parts of the mouth that help us speak, eat, and show emotions. They help protect the teeth and mouth. An example of a birth defect that can occur when the lips do not form correctly during pregnancy is cleft lip (opening in the upper lip).

The palate is the roof of the mouth. It is a structure made up of bone and soft tissue that separates your mouth from your nasal cavity (where you breathe through your nose). The palate helps you eat and speak by guiding food and air where they need to go. An example of a birth defect that can occur when the palate does not form correctly during pregnancy is cleft palate (opening in the roof of the mouth).

Teeth are hard, white structures in our mouths that help us chew and digest food. They are made mostly of a material called enamel, which is the hardest substance in our bodies. An example of a birth defect that can occur when the teeth do not form correctly during pregnancy is hyperdontia (extra teeth).

The external genitalia include the organs that are located outside of the body, like the clitoris or penis. These organs can have more than one function. For example, the penis helps removes urine from the body and release sperm. Examples of birth defects that can occur when the external genitalia do not form correctly during pregnancy are hypospadias (opening of the penis is on the underside instead of the tip) and clitoromegaly (larger than expected clitoris).

Miscarriage is defined as a pregnancy loss before 20 weeks. Miscarriage is common and can occur in any pregnancy for many different reasons. In the US, about 1 in every 7 pregnancies results in miscarriage.

Stillbirth is defined as a pregnancy loss after 20 weeks. In the US, about 1 in every 175 pregnancies results in a stillbirth.

Preterm Birth: Preterm birth is when a baby is born before 37 weeks of pregnancy. Babies born preterm are more likely to experience health problems. In the US, about 1 in every 10 babies is born preterm.

A term birth is when a baby is fully developed and ready to be born. Any delivery from 37 weeks of pregnancy up to 42 weeks is considered term. Babies born at term are less likely to experience health problems. In the US, about 9 out of every 10 babies is born to term.

During the all or none period, exposures are thought to either have an all (miscarriage) or none (no risk) effect.

There is a “critical period” during pregnancy when the risk of birth defects is highest. During this time, various factors such as exposures, genetics, and health conditions can influence the development of one or more body parts. Birth defects can range from mild to severe, potentially requiring surgery or leading to lifelong disabilities.