• Clinical science



Pregnancy begins with the fertilization of the ovum and subsequent implantation into the uterine wall. The duration of pregnancy is counted in weeks of gestation from the first day of the last menstrual period and on average lasts 40 weeks. Presumptive signs of pregnancy include amenorrhea, nausea and vomiting, and breast enlargement and tenderness. Pregnancy can be confirmed definitively via positive serum or urine hCG tests and detection of the embryo on ultrasound. Ultrasound is also used to determine the gestational age and date of delivery. Women experience several benign, physiological changes during pregnancy (e.g., peripheral edema, dyspnea, increased urinary frequency). Regular check-ups should be performed to detect possible high-risk pregnancies as well as fetal and maternal complications.

See also maternal complications during pregnancy.


  • Gravidity: the number of times a woman has been pregnant
  • Parity: the number of times a woman has given birth

Duration of pregnancy

  • The duration of pregnancy is counted in weeks of gestation from the first day of the last menstrual period
  • Normal duration of pregnancy: 40 weeks (280 days)
  • Preterm birth: live birth before the completion of 37 weeks (< 37 0/7) of pregnancy
  • Post-term birth: live birth after 42 weeks (> 42 0/7) of pregnancy


Stages of pregnancy

Pregnancy is divided into trimesters:

  • First trimester (week 1–13): highest risk of miscarriage
  • Second trimester (week 14–26)
  • Third trimester (week 27–40)
Day/week Stage Possible disorders
Day 0
Day 1–5
Day 6
Days 6–12
Week 3
  • Embryopathies: complex anomalies of individual organs
  • Particularly susceptible to teratogens
    • Infections
    • Drug toxicity
Weeks 3–8
  • Embryogenesis
  • Organogenesis: organ formation (chronological order)
    • CNS: The neural tube closes by week 4.
    • Heart: The heart begins to beat by week 4.
    • Ears and eyes
    • Upper and lower extremities
    • Teeth and gums
    • External genitalia (male/female characteristics)
From week 9
  • Fetogenesis: the fetus matures and grows.

The earlier disruptions take place during fetal development, the more complicated the resulting congenital anomalies!


Clinical signs of early pregnancy


Diagnosis of pregnancy

Confirmation of pregnancy

Transvaginal ultrasound findings
in normal pregnancy

Weeks after last menstruation β-hCG mlU/mL
Gestational sac 5 1500–2000
Yolk sac 6 2500
Fetal pole 7 5000
Fetal heartbeat 8 17000Determining the gestational age and date of delivery

The gestational age and date of delivery may be estimated using Naegele's rule or determined more accurately via ultrasound in the second and third trimester.

  • Gestational age: the age (in weeks and days) of the fetus calculated from the first day of the last menstrual period
  • Embryonic age: the age (in weeks and days) of the fetus calculated from the day of conception (fertilization)
  • Naegele's rule: used to calculate the date of delivery (due date)
    • First day of the last menstrual period + 7 days + 1 year - 3 months
    • Inaccurate if:
      • The date of the last menstrual period is uncertain or unknown.
      • The patient has irregular menstruation cycles.
      • The patient conceived while taking contraceptive pills.
  • Ultrasound: more accurate than Naegele's rule
    • Measurement of the crown-rump length (CRL) in the first trimester
    • Measurement of biparietal diameter, femur length, and abdominal circumference (starting at 14 weeks)
  • Fundal height during pregnancy: correlates with the gestational age


Physiological changes during pregnancy

Numerous physiological changes occur during pregnancy in order to support fetal growth and prepare for delivery.

Organ system Changes
  • Progesterone↓ vascular tone↓ peripheral vascular resistance
    • ↑ Cardiac output (by up to 40%)
    • Stroke volume (by ∼ 10–30%)
    • Heart rate (by ∼ 12–18 bpm)
    • Mean arterial pressure
  • Innocent systolic murmur
  • The apex beat is displaced upward.
  • Varicosity and edema of lower limbs
Mammary glands
  • ↑ Size
  • ↑ Size
Vulva and vagina

A physiological systolic murmur may be heard due to increased cardiac output and increased plasma volume!

Physiological hypercoagulability during pregnancy leads to an increased risk of thrombosis! Patients suffering from thrombophilia should receive adequate thrombosis prophylaxis!


Nutrition during pregnancy

Basic principles

  • Nutritional intake needs to be adapted to meet both the demands of the mother and the fetus.
  • Dietary recommendations:
    • Limit caffeine intake: daily recommended dose < 200 mg
    • Avoid alcohol and tobacco use throughout pregnancy
    • Avoid unwashed or uncooked foods
  • Supplementation of vitamins and minerals is recommended.
Supplementation Reason for increased demand Consequences of deficiency
Folic acid
  • Folic acid 0.4–0.8 mg/day: Ideally, women should begin intake 4 weeks prior to pregnancy and continue with this dose for at least the first 2–3 months of pregnancy.
Vitamin B12
  • Recommended for pregnant women who adhere to a vegan diet
  • 30–60 mg/day
  • 1000–1300 mg/day (50% increase)
  • Increased fetal demand
  • Lactation
  • 250 μg/day
  • Increased metabolism and excretion

Recommended weight gain during pregnancy

  • The recommendations are determined by the BMI prior to the pregnancy:
    • BMI < 18.5 (underweight): 28–40 lb
    • BMI 18.5–24.9 (normal weight): 25–35 lb
    • BMI 25–29.9 (overweight): 15–25 lb
    • BMI ≥ 30 (obese): 11–20 lb
  • Average recommended daily calorie intake:
    • First trimester: 2200 kcal
    • Second and third trimester: 2500 kcal
  • Regular physical activity is recommended (contact sport should be avoided).


Physical activity during pregnancy

  • Regular physical activity (i.e., aerobic and strength-training exercise) is considered beneficial and is recommended before, during, and after pregnancy.
  • Careful evaluation of medical and obstetric disorders is necessary before recommending any exercise program.
  • Modifying normal exercise routines may be necessary; activities considered unsafe should be avoided.
Safe and unsafe activities during pregnancy
Safe activities*
  • Walking
  • Running or jogging***
  • Swimming
  • Stationary cycling
  • Pilates
  • Yoga**
  • Racquet sports***
  • Strength training***
Unsafe activities
  • Contact sports (e.g., soccer, basketball)
  • Activities associated with a high risk of falling (e.g., snow skiing, water skiing, gymnastics, surfing)
  • Hot yoga
  • Hot Pilates
  • Skydiving
  • Scuba diving

* In pregnancies without complications as determined by an obstetrician

** Avoid positions associated with ↓ venous return

*** May be considered safe in women who took part in these exercises before the pregnancy and as determined by an obstetrician

Physical activity should be discontinued if the patient presents with any of the following: antepartum or postpartum hemorrhage, uterine contractions, amniotic fluid leakage, chest pain, dyspnea before exertion, dizziness, headaches, calf pain/swelling, and/or muscle weakness with impaired balance!


High-risk pregnancies

Early identification of high-risk pregnancies is vital in order to prevent the occurrence of maternal and fetal complications, which are associated with high morbidity and mortality rates. High-risk pregnancies require regular prenatal care to monitor and support the pregnant mother (see prenatal care).

Risk factors for a complicated pregnancy


Fetal complications during pregnancy




Preconception counseling

General principles

  • All women with reproductive potential should be asked about their intention to become pregnant; by their healthcare provider; all those planning to become pregnant should be counseled.
  • Prenatal counseling aims at:
    • Identifying and addressing any modifiable risk factors for the woman, future pregnancy, and the fetus
    • Educating a woman about the risk factors and options for their reduction or elimination

Medical conditions

Lifestyle modifications

  • Recommend maintaining a normal body weight prior to conception.
  • Encourage regular moderate-intensity exercise.
  • Nutrition
    • Screen for sufficient intake of macronutrients and micronutrients such as calcium, iron, vitamin B12, vitamin B, vitamin D, etc.
    • Ensure vitamin A is not taken in excess.
    • Recommend intake of 400 mg of folic acid daily, ideally 4 weeks prior to pregnancy and continue for at least the first 2–3 months of pregnancy.
  • Substance use
    • Screen all women for alcohol, tobacco, and drug use and educate about associated adverse effects for the maternal and fetal pregnancy outcomes.
    • Provide assistance and/or refer to the proper specialists for smoking cessation and alcohol and drug use discontinuation.

Management of exposures

  • Exposures to harmful agents
    • Advise patients to assess workspace and household for potentially harmful agents (e.g., heavy metals, solvents, chemicals, etc.).
    • Educate the patient about the avoidance of harmful agents and refer them to occupational medicine programs as needed.
  • Exposure to violence
    • Screen for family violence at each visit.
    • In case ongoing abuse is identified, providers should:
      • Provide community resources to the patient.
      • Report the case as appropriate.

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last updated 03/27/2020
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