The acronym “US OB 14 weeks” refers to an Ultrasound Obstetrics examination performed at 14 weeks of gestation. This time point marks a significant transition, as it is the start of the second trimester of pregnancy. The second trimester is often a period of renewed energy for the pregnant person and one of rapid, visible growth for the fetus. An ultrasound at this stage provides healthcare providers with critical information about fetal development and helps confirm the established due date.
Fetal Development Milestones at 14 Weeks
At 14 weeks, the developing human is formally recognized as a fetus, having completed the embryonic stage where all major organ systems were initially formed. The fetus is approximately 3.5 inches long from crown to rump, roughly the size of a lemon, and weighs around 1.5 ounces. This period initiates a phase of extensive growth and refinement of the body systems.
The framework of the skeleton is rapidly transitioning as cartilage begins to harden into bone, a process known as ossification. This hardening is particularly noticeable in the long bones of the limbs, which appear whiter and brighter on the ultrasound image. The neck is now more defined and the head is becoming more proportional to the rest of the body.
Internal organs are beginning to function. The fetal thyroid gland is maturing and starting to produce hormones, while the liver produces bile. The spleen takes on the important role of producing red blood cells, which it will maintain until the bone marrow assumes this function later in pregnancy.
The fetus demonstrates increasingly coordinated movements, though they are usually too subtle for the mother to feel. Due to developing facial muscles, the fetus can squint, frown, grimace, and even make sucking motions. The kidneys are also fully functional, with the fetus swallowing amniniotic fluid and passing it out as urine, a process that helps maintain the fluid volume.
Fine, downy hair called lanugo starts to emerge across the body, serving a protective function for the skin. The external genitalia have fully developed, although the small size and fetal position can still make sex determination challenging on an ultrasound at this specific week.
Maternal Physiological Changes in the Second Trimester
The start of the second trimester often brings a noticeable improvement in the pregnant person’s well-being. Symptoms like severe morning sickness and extreme fatigue, common in the first trimester, begin to subside. The reduction in nausea is often accompanied by a significant increase in appetite and energy levels.
The uterus has grown significantly and is starting to rise out of the pelvic cavity and into the lower abdomen. This upward expansion is what causes the “baby bump” to become more visible, though the exact timing varies greatly among individuals. The shift in uterine position also often relieves pressure on the bladder, leading to less frequent urges to urinate compared to the early weeks.
Increased blood volume, which can rise by up to 40% during the second trimester, supports the growing placenta and fetus. This hyperdynamic circulation can cause the mucous membranes to swell, sometimes resulting in nasal congestion or occasional nosebleeds. Hormonal changes also contribute to breast enlargement and the potential for a yellowish, thick fluid called colostrum to be produced, which is an early form of milk.
Interpreting Ultrasound Measurements and Screening
The ultrasound examination at or near 14 weeks is performed to assess fetal growth and development, as well as to examine the placenta and amniotic fluid. At this stage, the fetus is too large and curled for the Crown-Rump Length (CRL) measurement, previously used in the first trimester, to provide the most accurate dating.
The sonographer transitions to using multi-parameter measurements, collectively known as fetal biometry, to confirm gestational age and monitor growth. These measurements are used in combination to provide a comprehensive estimate of the fetus’s age and size. The standard measurements taken include:
- The Biparietal Diameter (BPD), which is the distance between the two sides of the fetal head.
- The Head Circumference (HC), which assesses the overall size of the head.
- The Femur Length (FL), which measures the length of the longest bone in the body.
- The Abdominal Circumference (AC), which is measured to evaluate the size of the fetal abdomen and track weight gain.
The 14-week scan also provides an opportunity to evaluate the position of the placenta, noting whether it is situated normally or if it covers the cervix, a condition called placenta previa, which requires future monitoring. While some anatomical structures are checked, this is not the comprehensive anatomy survey. That scan is typically scheduled later, between 18 and 22 weeks, when the fetus is larger and structures are more easily visualized. For those who have not yet completed genetic screening, the quad screen blood test is offered between 15 and 20 weeks to assess risk for certain chromosomal conditions and neural tube defects.
Common Appointments and Next Steps
Following the 14-week mark, prenatal visits typically settle into a routine schedule of once every four weeks until the 28th week. At each appointment, routine checks will include monitoring the pregnant person’s weight and blood pressure, along with a urine sample to screen for protein and sugar. Healthcare providers will also listen to the fetal heart rate using a Doppler device.
This period is a good time to focus on maintaining a balanced diet and regular, moderate exercise, which supports both maternal and fetal health. Providers advise continuing prenatal vitamins and discussing any new or concerning symptoms immediately. Preparing for the comprehensive fetal anatomy scan, which will occur in the coming weeks, is the next major step in monitoring the pregnancy.