Week 16 of pregnancy: changes in the fetus and mother

By (embryologist), (gynecologist) and (psychologist).
Last Update: 02/22/2022

The 16th week of pregnancy is the last week of the fourth month of gestation and many changes occur in the baby.

The uterus has already reached a considerable size and the blood vessels are compressed, so the woman may experience swollen feet, tired legs, varicose veins, etc. For these reasons, week 16 of gestation is a good time for the mother-to-be to start practicing moderate sports.

However, the mother should be happy and calm because if she has not yet felt the baby, she will soon feel it moving inside her.

Changes in the fetus

From the 16th week of pregnancy, the baby begins to develop the bones of the ear and the back muscles are gaining volume and strength. Therefore, it will be able to respond to stimuli and recognize sounds. In addition, the brain acquires the function of limb control by increasing sensory capacity.

In general, the baby begins to have more balanced proportions, the eyes appear centrally located and the ears ascend to occupy their almost definitive position.

A 16-week-old baby measures approximately 11-13 cm and can weigh 80-100 g. In addition, it has a head diameter (BPD) of 33 mm and a femur length of 21 mm.

In relation to the changes that occur in the baby during the 16th week of pregnancy, the following can be highlighted:

  • Hair appears on the eyebrows and on the upper lip. This hair falls out after birth.
  • Nails form on the fingers and toes.
  • Development of epidermal folds on the palms of the hands and fingers. In addition, the baby already has a fingerprint.
  • The legs are longer than the arms and with great mobility.
  • Knees, ankles, elbows, wrists, and fingers can be differentiated. This development allows the baby to be more functional and to be able to open and close his hands, move his legs, etc.
  • The eyelids are closed but begin to make small movements of the eyes from side to side.
  • Hematopoiesisbegins. It is the process of formation, development, and maturation of the elements that make up the blood (red blood cells, leukocytes, platelets, etc.). Thus, the baby will have its own blood and its heart will be in charge of pumping it.
  • The pressure reflex develops so that the baby is able to grasp the feet or the umbilical cord. It also allows you to keep your fists closed.

Symptoms and changes in the mother

The 16th week of gestation, along with the rest of the weeks that make up the fourth month of pregnancy, is one of the best times for the mother because she has already become accustomed to all the hormonal change. Despite this, the expectant mother will continue to experience changes and will notice some of the symptoms listed below.

  • The size of the uterus increases. The uterine height from the fundus to the pubis measures 14 cm. The mother may feel the lower part of the abdomen hard and there may be pinching or discomfort in the sides of the uterus. This is not a cause for concern, as it is the adaptation of the ligaments to the new uterine shape.
  • Disappearance of morning sickness and vomiting.
  • You may begin to notice certain movements of the baby, but usually a few weeks later (around the 18th week of gestation).
  • Night cramps due to involuntary contractions of the baby.
  • Constipation due to the combination of pregnancy hormones and the pressure of the uterus on the intestines.
  • Increased breast volume and sensitivity.
  • Increased appetite, as the baby is more developed and requires more energy.
  • Appearance of cavities and bleeding gums due to vitamin deficiency.
  • Back pain due to the volume of the abdomen.
  • Increased salivation in relation to elevated estrogen levels. This is known as hypersialorrhea o ptyalism.

Pregnancy control: diagnostic tests

When a woman becomes aware that she is pregnant, it is important to keep control of her pregnancy to avoid risks.

Between the 16th and 20th week of gestation, a few years ago doctors used to determine alpha-fetoprotein (AFP). Increased AFP indicates possible neural tube defects (spina bifida); whereas decreased levels of this protein are indicative of Down syndrome.

However, at present this test is not recommended by the Spanish Society of Obstetrics and Gynecology (SEGO) because it provides numerous false positives, that is, it reports some defects that are not real.

Therefore, the most recommended tests for the 16th week of pregnancy are:

  • Routine blood pressure control, blood tests, weight control, etc.
  • Additionally, there are women who go privately to have an ultrasound to find out the sex of the baby in the 16th week of pregnancy, since generally this ultrasound isn´t performed until the 20th week of pregnancy.
  • Screening for asymptomatic bacteriuria. During pregnancy there is an alteration in the ability to empty the bladder and the properties of urine are modified, so it is more common for microorganisms to grow and cause infections. This screening is also recommended at 12 weeks of gestation.
  • Amniocentesis This is an invasive prenatal test that involves the extraction of amniotic fluid by puncturing the mother's abdominal wall. The extracted amniotic fluid will be analyzed and the baby's karyotype, that is, its set of chromosomes, will be obtained. In this way, chromosomal alterations can be detected.


In the 16th week of pregnancy, it is recommended to do some type of moderate physical exercise or sport. This stimulates blood circulation and reduces lumbar discomfort. The sport most recommended by doctors during pregnancy is swimming.

In addition, abundant water consumption will promote hydration and blood circulation. Feeding must be adequate to ensure the correct supply of nutrients to the baby. With a healthy, varied, and balanced diet, the mother will feel better and will favor fetal development.

In any case, all the instructions given by the obstetrician will be followed at all times to avoid risks that may occur during pregnancy.

FAQs from users

By Marta Barranquero Gómez B.Sc., M.Sc. (embryologist).

Amniocentesis is an invasive prenatal technique that is recommended from the age of 35; while from the age of 40 it becomes a mandatory test.

This technique will also be recommended in women who have had a history of repeated miscarriages or other previous pregnancies with chromosomal alterations, regardless of age.

At 16 weeks of pregnancy, can the sex of the baby be seen by ultrasound?

By Óscar Oviedo Moreno M.D. (gynecologist).

From 12 weeks the sign of the genital tubercle can be seen. From the 16th week onwards, the male and female genitalia can be visualized as long as the fetus is well-positioned.

What month is the 16th week of pregnancy?

By Marta Barranquero Gómez B.Sc., M.Sc. (embryologist).

The 16th week of gestation corresponds to the last week of the fourth month of pregnancy. This week is therefore included in the second trimester of pregnancy.

In the case of twin pregnancy, are the changes that occur at 16 weeks similar?

By Marta Barranquero Gómez B.Sc., M.Sc. (embryologist).

Yes, the mother and fetuses will experience the same changes as with a singleton pregnancy. With a twin pregnancy it is possible that in the 16th week you may notice more marked movements of the babies.

If you want to know how the pregnancy is going to evolve, we invite you to read the article of the next week of gestation: 17th week of pregnancy.

We have also talked about amniocentesis, so if you want to learn more about this technique we recommend you to visit the following link: Amniocentesis: what are its indications and possible risks?

We make a great effort to provide you with the highest quality information.

🙏 Please share this article if you liked it. 💜💜 You help us continue!


Cabanyes, J. El comportamiento fetal: una ventana al neurodesarrollo y al diagnóstico temprano. Septiembre 2014. Pediatría Atención Primaria vol.16 (63). DOI: 10.4321/S1139-76322014000400012

Health Service Excecutive (Feidhmeannacht na Seirbhíse Sláinte) (2010). A guide to your pregnancy month by month. Breastfeeding.ie - Your breastfeeding support network. Order Code: HPM00341

Zheng, Y, Zhang, C, Weisskopf, M, Williams, PL, Parsons, PJ, Palmer, CD, Buck Louis, GM, James-Todd T. A prospective study of early pregnancy essential metal(loid)s and glucose levels late in the second trimester. J Clin Endocrinol Metab. 2019 May 16. DOI: 10.1210/jc.2019-00109

FAQs from users: 'At what age is amniocentesis recommended?', 'At 16 weeks of pregnancy, can the sex of the baby be seen by ultrasound?', 'What month is the 16th week of pregnancy?' and 'In the case of twin pregnancy, are the changes that occur at 16 weeks similar?'.

Read more

Authors and contributors

 Marta Barranquero Gómez
Marta Barranquero Gómez
B.Sc., M.Sc.
Graduated in Biochemistry and Biomedical Sciences by the University of Valencia (UV) and specialized in Assisted Reproduction by the University of Alcalá de Henares (UAH) in collaboration with Ginefiv and in Clinical Genetics by the University of Alcalá de Henares (UAH). More information about Marta Barranquero Gómez
License: 3316-CV
 Óscar Oviedo Moreno
Óscar Oviedo Moreno
Bachelor's Degree in Medicine & Surgery from the University of Caldas (Colombia). Specialist in Internal Medicine by the Pontificia Universidad Javeriana of Bogotá. Degree standardized in Spain in 2003. Specialist in Gynecology & Obstetrics from the Complutense University of Madrid, with residence at Hospital Clínico Universitario San Carlos de Madrid. Expert in Reproductive Medicine and Certification in Obstetric-Gynecologic Ultrasound (levels I, II and III). More information about Óscar Oviedo Moreno
License: 282858310
Adapted into english by:
 Cristina  Algarra Goosman
Cristina Algarra Goosman
B.Sc., M.Sc.
Graduated in Psychology by the University of Valencia (UV) and specialized in Clinical Psychology by the European University Center and specific training in Infertility: Legal, Medical and Psychosocial Aspects by University of Valencia (UV) and ADEIT.
More information about Cristina Algarra Goosman
Member number: CV16874

Find the latest news on assisted reproduction in our channels.