Intrauterine Pregnancy:  Symptoms, Meaning, how to detect & What to expect

What is Intrauterine Pregnancy (IUP)?

IUP refers to intrauterine pregnancy, which is a medical term that simply means a pregnancy in which the fertilized ovum gets implanted in the wall of the uterus (generally the fundus region of the uterus, which is the upper part of the uterus). That is the most common site for the implantation of the fertilized egg.

After ovulation, within 48 hours, the egg can be fertilized by sperm in the isthmus part of the fallopian tube (almost the mid part of the fallopian tube), and from there, the fertilized egg travels through the fallopian tube and gets implanted in the uterus. Therefore, IUP refers to a pregnancy in which the fertilized ovum implants and develops within the uterine cavity.

How intrauterine pregnancy happens?

Till now, you know intrauterine pregnancy means fertilization of the egg and implantation of the zygote (fertilized egg) in the uterine lining (endometrial lining of the uterus). Let’s see now, step by step—

1. Fertilization – During the ovulatory phase of the menstrual cycle, ovulation occurs — a mature egg is released from the ovary and taken up by the fallopian tube. In the mid part of the fallopian tube, this egg can be fertilized by sperm and form a zygote.
2. Early development – While moving toward the uterus, this zygote undergoes multiple cellular divisions and forms a blastocyst — this refers to a small group of rapidly dividing cells.
3. Implantation – This blastocyst reaches the uterine lining and embeds itself into the endometrial lining of the uterus.
4. Site – In most healthy pregnancies, the best site for the implantation of the blastocyst is the fundal region of the uterus because it offers better blood supply and sufficient space for the growing baby.

Signs & Symptoms of Intrauterine Pregnancy

Mainly, the signs and symptoms of intrauterine pregnancy generally involve the early signs and symptoms of a normal pregnancy unless there is any complication of intrauterine pregnancy, i.e.,

• Pregnancy of uncertain viability
• Chemical pregnancy
• Spontaneous abortion, etc.

Symptoms:

• Nausea
• Vomiting
• Dizziness
• Shortness of breath
• Increased frequency of urination or peeing
• Constipation
• Weakness & fatigue
• Missed period (Amenorrhea)
• Mild spotting (Implantation bleeding)

Stages of Intrauterine Pregnancy:

The total gestational period is 9 months, or 280 days, or 40 weeks after the last menstrual period. The implantation of the fertilized egg in the uterine wall and the duration of development of the fetus/growing baby in the uterine cavity is further divided into 3 stages:

• 1st trimester – Includes the first 12 weeks of pregnancy or the first 3 months of pregnancy.
• 2nd trimester – Includes the 13th to 24th weeks of pregnancy or the 4th to 6th months of pregnancy.
• 3rd trimester – Includes the 25th week up to the delivery of the baby, or from the 7th month to the 9th month of pregnancy.

1st Trimester

It includes the first 3 months of intrauterine pregnancy from the LMP (last menstrual period), which means it includes the first 12 weeks of pregnancy from LMP.

Maternal changes:

i) Morning sickness (nausea & vomiting)
ii) Missed period
iii) Increased frequency of urination
iv) Mild lower abdominal pain
v) Food cravings
vi) Headaches
vii) Breasts become tender and fuller

Fetal development:

• The zygote develops into an embryo through rapid cell division and forms major body systems and organs.
• Basic features of the face and limbs develop as buds.
• At the end of the first trimester, the fetus weighs approximately 20 grams.

2nd Trimester:

It includes the 4th to 6th month of intrauterine pregnancy, which means it covers the 13th week to the 26th week of the gestation period.

Maternal changes:

• Less nausea & vomiting (morning sickness)
• Size of the abdomen increases as the uterus grows because of the developing baby
• The first fetal movement can be felt by the mother during the 5th month of pregnancy, often described as a “butterfly feeling” in the stomach by some pregnant women
• As the abdomen grows, a few stretch marks may appear on the skin due to stretching
• Mild swelling in the feet may occur
• Increased frequency of urination
• Increased appetite

Fetal changes:

• Organs and bones start to develop
• Features of the face become clearer, and the baby begins to move
• By the end of this trimester, the baby weighs approximately 600–700 grams

3rd Trimester:

It includes the last 3 months of intrauterine pregnancy, which means it covers the period from the 27th week up to the delivery of the baby (generally the 40th week of intrauterine pregnancy).

Maternal changes:

• Increase in the size of the abdomen as the uterus reaches its maximum size, causing a noticeable bump
• Increased fatigue and sleep disturbances due to discomfort and frequent urination
• As the uterus enlarges, it pushes the diaphragm upwards, reducing space for breathing, which may cause shortness of breath
• Back pain
• Leg cramps
• Heartburn and indigestion
• Linea Nigra (skin changes): A dark vertical line on the abdomen
• Increased appetite

Fetal changes:

• The lungs and brain of the baby mature
• The skin of the baby smoothens as fat builds up
• At the end of this trimester, the baby weighs around 2.5 kg to 3.5 kg

How to Confirm Intrauterine Pregnancy

After ovulation, the egg gets fertilized within 48 hours and moves toward the uterine cavity, where it gets implanted. The first symptom of intrauterine pregnancy (IUP) is a missed period (amenorrhea). After this, a woman may suspect that she is pregnant.

For provisional confirmation of pregnancy, a UPT (Urine Pregnancy Test) can be done by the woman herself. Other tests include blood tests, ultrasound, etc.

Blood Test:

This test can come positive 6–8 days after ovulation, around the time of implantation of the fertilized egg. It measures the level of β-hCG (hormone) in the blood serum. However, this test does not detect the location of implantation of the fertilized egg.

UPT (Urine Pregnancy Test):

This test can come positive after approximately 2 weeks of gestation. It can be performed individually by the expecting woman herself. This test detects the increased level of β-hCG in the urine. However, it does not determine the location of implantation of the fertilized egg (whether it is an intrauterine pregnancy or an ectopic pregnancy).

Transvaginal Ultrasound (TVU):

This test is done during 4.5 to 6.5 weeks of gestational age. This test is the gold standard test for confirmation of intrauterine pregnancy. Ultrasound can scan the sonographic image of the gestational sac, yolk sac, and heart activity etc., which helps to rule out other complicated conditions such as ectopic pregnancy, stillbirth, spontaneous abortion etc.

• Gestational sac can be visible around 4.5 to 5 weeks of gestational period.
• Yolk sac can be visible around 5 to 5.5 weeks of gestational period.
• Fetal heart activity can be visible around 6 to 6.5 weeks of gestational period.

TVU also helps in ruling out pregnancy of uncertain viability in which implantation is normal but the heartbeat of the baby is not there.

Research Evidence: Gold Standard for Confirming Intrauterine Pregnancy

Clinical research consistently shows that transvaginal ultrasound (TVUS) combined with serum β-hCG measurement provides the highest diagnostic accuracy in early pregnancy. A gestational sac is typically visible at 4.5–5 weeks, a yolk sac at 5–5.5 weeks, and fetal cardiac activity by 6–6.5 weeks of gestation.

If β-hCG levels rise above expected thresholds and no intrauterine sac is visible, ectopic pregnancy must be suspected. Studies conclude that relying on symptoms or urine testing alone is insufficient — imaging confirmation is essential for safe early pregnancy care.

Research Sources:

• Blaivas M, et al. Role of ultrasound in the evaluation of first-trimester pregnancy. Journal of Ultrasound in Medicine, 2019.
• Ghosh S, et al. Early diagnosis of ectopic pregnancy based on algorithmic approaches and biomarkers. Journal of Embryology and Developmental Biology.

How to Rule Out Ectopic Pregnancy:

Ectopic pregnancy is the pregnancy in which the fertilized egg gets implanted at sites other than the uterus, which is not a common pregnancy site. The general site of ectopic pregnancy is the fallopian tube. Only on the basis of symptoms, we can’t rule out whether the pregnancy is intrauterine or ectopic. Transvaginal Ultrasound can help us to rule out ectopic pregnancy.

If there is any chance that the pregnancy is ectopic, you need to consult a gynaecologist to avoid further complications, i.e., a growing mass in the fallopian tube may rupture the fallopian tube, which may lead to sterility for women. To avoid all of this, you should pay proper attention to the ANC & prenatal care and follow a proper visiting schedule with your doctor.

Case Insight: Why Early Ultrasound Matters in Intrauterine Pregnancy

A 29-year-old woman at 6 weeks of gestation presented with mild abdominal discomfort and a positive pregnancy test. Transvaginal ultrasound confirmed a gestational sac and yolk sac inside the uterine cavity, consistent with a normal intrauterine pregnancy (IUP). Serum β-hCG levels matched the expected gestational age.

This case reinforces a key clinical principle: only ultrasound can confirm the location of implantation. A urine test confirms pregnancy hormonally, but it cannot rule out ectopic pregnancy. Early transvaginal scanning ensures proper diagnosis and prevents delayed detection of potentially life-threatening complications.

Reference:
Blaivas M. et al. Role of ultrasound in the evaluation of first-trimester pregnancy. 2019. Journal of Ultrasound in Medicine, 2019

Symptoms which may appear in ectopic pregnancy:

• Severe abdominal pain (sharp)
• Light vaginal bleeding can be seen
• Shoulder pain
• Signs of shock due to any internal bleeding

Complications of intrauterine pregnancy:

• Placenta previa
• Preeclampsia
• Preterm labor
• Miscarriage
• Gestational diabetes
• Gestational hypertension

Lifestyle changes during pregnancy:

• Eat a balanced diet which is rich in iron, calcium, folate, and protein
• Always stay hydrated and limit caffeine
• Avoid alcohol, smoking, and recreational drugs
• Maintain adequate rest and adopt habits of deep breathing or meditation

Frequently Asked Questions (FAQs):

Note from Femwise Health

You should better understand intrauterine pregnancy now, and TVU helps with confirmation of IUP and ruling out a few conditions, i.e., ectopic pregnancy, stillbirth, PUV, etc. A pregnancy may be associated with many risk factors; therefore, one should take proper ANC & prenatal care.

Glossary:

• Zygote – It forms after fertilization of the egg with sperm
• Blastocyst – Early stage of the embryo which gets implanted into the uterus
• Endometrium – Innermost lining of the uterus where the embryo gets implanted
• Fundus – Uppermost dome part of the uterus, which is the common site for the implantation of the embryo
• Gestational age – Age of the pregnancy from the last date of the menses
• Gestational sac – A fluid-filled sac around the embryo; its first visible sign under ultrasound of the pregnancy