Human Placental Lactogen (hPL) Test: Placenta Function, Pregnancy Hormone, and Results

Human placental lactogen, or hPL, is a protein hormone made by the placenta that helps shift maternal metabolism so the fetus has a steady supply of nutrients. It is also called human chorionic somatomammotropin, or hCS. Maternal blood levels rise as placental mass increases, becoming highest late in pregnancy and falling rapidly after delivery. Although hPL was once investigated as a blood marker of placental function and fetal growth, clinicians do not routinely use a stand-alone hPL test in modern prenatal care. Ultrasound growth measurements, amniotic-fluid assessment, Doppler studies, fetal surveillance, glucose testing, and other condition-specific tests provide more actionable information. When hPL is measured, the result must be matched to gestational age, laboratory method, fetus number, maternal metabolic health, and the reason for testing. A low or high value cannot diagnose placental insufficiency, growth restriction, diabetes, or fetal well-being by itself. Unexpected results usually prompt review of the full clinical picture rather than treatment directed at the hormone level.

• hPL is produced mainly by placental syncytiotrophoblast cells and rises through pregnancy as the placenta grows.
• The hormone promotes maternal insulin resistance and fat use, helping preserve glucose for the fetus.
• hPL testing is uncommon in routine prenatal care and has no single universal pregnancy reference range.
• Low hPL may reflect small placental mass or placental dysfunction, but it is not specific enough for diagnosis.
• High hPL can occur with larger placental mass, multiple pregnancy, or altered glucose metabolism.

Table of Contents

• What Human Placental Lactogen Is
• How hPL Supports Pregnancy Metabolism
• When an hPL Test May Be Ordered
• Testing, Timing, and Reference Values
• Low hPL Results
• High hPL Results
• hPL, Glucose, and Fetal Growth
• Better Ways to Assess the Placenta and Fetus

What Human Placental Lactogen Is

Human placental lactogen is a single-chain peptide hormone produced primarily by the syncytiotrophoblast, the outer placental cell layer that interfaces with maternal blood. Its structure resembles both growth hormone and prolactin, which explains its alternative name, human chorionic somatomammotropin. “Lactogen” refers to prolactin-like effects, while “somatomammotropin” reflects growth hormone-like and breast-related actions.

hPL becomes detectable early in pregnancy, often around the fifth or sixth week. Its concentration generally rises with gestational age and placental size, reaching its highest levels in the third trimester. Unlike hCG, which peaks near the end of the first trimester and then falls, hPL continues to increase toward term. The maternal circulation receives most of the hormone, so its strongest recognized actions are on the pregnant person’s tissues rather than directly on the fetus.

After the placenta is delivered, hPL clears quickly, often becoming undetectable within a day. This rapid fall confirms that the placenta is the major source. A persistent result after pregnancy would be unusual and should first raise questions about assay timing, retained placental tissue, or laboratory interpretation rather than being treated as a routine postpartum marker.

The amount of hPL in blood has historically been viewed as a rough reflection of functioning placental tissue. Larger placentas and multiple pregnancies often produce more, while a small or damaged placenta may produce less. However, production is not a direct measure of oxygen transfer, blood flow, fetal neurologic status, or placental reserve. Two placentas with similar mass can function differently, and one blood sample cannot show that difference reliably.

hPL is one member of a broad placental endocrine system. hCG supports the corpus luteum early; progesterone and estrogens maintain pregnancy and coordinate tissue changes; placental growth hormone, corticotropin-releasing hormone, and other signals alter maternal physiology. A pregnancy hormone panel does not replace obstetric assessment because no collection of hormone numbers directly establishes fetal well-being.

How hPL Supports Pregnancy Metabolism

Pregnancy requires the maternal body to support its own energy needs while continuously transferring glucose, amino acids, fatty acids, oxygen, and minerals to the fetus. hPL helps organize this adaptation, especially in the second half of pregnancy.

One major effect is reduced maternal insulin sensitivity. Insulin normally helps muscle and fat cells take up glucose. As hPL and other placental hormones rise, maternal tissues become less responsive, leaving more glucose in circulation for placental transfer. The pancreas usually compensates by making more insulin. This physiologic insulin resistance is expected; gestational diabetes develops when insulin secretion cannot keep pace with the increased demand.

hPL also promotes lipolysis, the release of fatty acids from maternal fat stores. Maternal tissues can burn more fatty acids for energy, which helps spare glucose for the fetus. During fasting, this shift becomes more pronounced. The adaptation is useful but can increase circulating triglycerides and ketones, particularly during prolonged fasting or illness.

Additional proposed actions include:

• supporting pancreatic beta-cell adaptation and insulin secretion;
• influencing maternal insulin-like growth factor pathways;
• helping prepare breast tissue for lactation;
• contributing to protein and amino-acid metabolism;
• participating in maternal bone and calcium adaptation through prolactin-receptor pathways.

hPL is not the only hormone producing these effects. Placental growth hormone, progesterone, estrogen, cortisol, prolactin, adipokines, inflammatory signals, and maternal body composition all contribute. Describing hPL as “the cause” of pregnancy insulin resistance oversimplifies a coordinated system.

The fetal benefit is indirect. By redirecting maternal fuel use, the placenta maintains a gradient that favors transfer of glucose and other nutrients. Excess maternal glucose can also cross the placenta and stimulate fetal insulin, encouraging excessive growth. That is why normal endocrine adaptation and gestational diabetes can look like points on a continuum, although hPL measurement itself does not diagnose where a patient falls on that continuum.

When an hPL Test May Be Ordered

Routine prenatal panels usually do not include hPL. Decades ago, serial hPL testing was studied as a way to detect placental insufficiency, fetal growth restriction, fetal compromise, or impending pregnancy loss. Its clinical role diminished because the marker varies widely and modern ultrasound and surveillance methods provide more direct information.

An hPL test may still appear in several settings:

• specialized research on placental hormones, diabetes, obesity, or fetal growth;
• evaluation by an endocrine or maternal-fetal medicine team using a nonstandard protocol;
• investigation of a suspected placental disorder when other findings are already abnormal;
• legacy laboratory panels or regions where older placental-function testing remains available;
• study of placental tissue or hormone production rather than routine patient management.

The reason for ordering should be clear. A clinician who requests hPL should be able to explain what decision the result could change. If the answer is simply “to check the placenta,” ask which validated reference interval and follow-up pathway will be used.

hPL is not an appropriate test to confirm pregnancy. For that, clinicians use hCG blood testing. It is not a screen for trisomy 21, neural tube defects, or structural anomalies. It also is not a substitute for glucose screening at 24–28 weeks or earlier risk-based diabetes testing.

A patient may encounter hPL in older medical records. In that context, an isolated result should be interpreted using the standards and gestational dating from that pregnancy. Applying an old value to current care without knowing the assay is unlikely to be useful.

Because hPL testing is uncommon, insurance coverage and laboratory availability can be limited. Samples may be sent to a reference laboratory, extending turnaround time. Before testing, confirm whether the test is clinically validated for the intended purpose and whether a more established assessment would answer the question better.

Testing, Timing, and Reference Values

The test uses maternal serum or plasma from a routine venous blood draw. Fasting is generally not necessary for hPL itself, though it may be required if glucose, insulin, lipids, or another metabolic test is collected at the same visit. Standard blood-draw effects include brief pain, bruising, and occasional lightheadedness.

Timing is central because hPL rises substantially as pregnancy advances. A concentration cannot be labeled high or low without the exact gestational age and the laboratory’s own method-specific interval. Published values may be reported in micrograms per milliliter, milligrams per liter, or other units. Since 1 microgram per milliliter equals 1 milligram per liter, the numbers can look similar even though the labels differ.

Older references often describe concentrations increasing from very low levels in early pregnancy to several micrograms per milliliter late in pregnancy. Some sources cite broad third-trimester values up to roughly 5–25 micrograms per milliliter near term. These figures are educational, not universal clinical cutoffs. Assay antibodies, calibration, sample handling, and population differences can shift the interval.

Interpretation requires several details:

Factor	Why it changes interpretation
Gestational age	hPL normally rises as pregnancy progresses
Number of fetuses	More placental tissue often produces more hPL
Placental mass	Larger mass tends to correlate with higher levels
Maternal diabetes or obesity	Metabolic state can alter associations and confound the result
Smoking and vascular disease	May affect placental growth and hormone production
Laboratory assay	Reference intervals are method specific
Serial versus single sample	A trajectory may be more informative than one value, though still limited

A single result should not be compared with an online range or a value from another pregnancy. Even serial results are not interpreted by a simple “doubling” rule. A plateau near term can be physiologic. Apparent changes may also reflect assay variation.

When a value is unexpected, first verify the pregnancy dating, units, specimen, and reference interval. Then examine ultrasound growth, placental appearance, blood pressure, glucose status, symptoms, and other clinical findings before assigning significance.

Low hPL Results

A low hPL result may indicate less hormone-producing placental tissue, but the finding is nonspecific. It can occur with incorrect gestational dating, a small placenta, fetal growth restriction, placental vascular disease, pregnancy loss, or certain maternal and fetal conditions. A single low result can also reflect normal variation or laboratory error.

Potential explanations include:

• gestational age earlier than recorded;
• fetal growth restriction associated with a small or poorly functioning placenta;
• preeclampsia or other placental vascular disease;
• placental infarction, abruption, or reduced viable placental mass;
• fetal demise or a failing pregnancy, depending on gestational stage and other findings;
• maternal smoking, undernutrition, or disease associated with impaired placental growth;
• specimen or assay problems.

Low hPL cannot tell whether the fetus is currently hypoxic. It does not measure placental blood flow, and it cannot distinguish a constitutionally small healthy fetus from growth restriction. If fetal size is a concern, serial ultrasound estimates, abdominal circumference growth, amniotic-fluid volume, and umbilical-artery Doppler are far more informative.

In early pregnancy, hPL is naturally low, so it is especially poor as a stand-alone viability marker. Pain or bleeding should be evaluated with ultrasound and hCG-based protocols rather than hPL. Later in pregnancy, reduced fetal movement requires same-day obstetric assessment; waiting for a hormone result is unsafe.

A low result does not have a direct replacement treatment. There is no hPL supplement used to correct placental function. Management targets the underlying condition: blood-pressure control, diabetes management, smoking cessation support, fetal-growth monitoring, antenatal corticosteroids when preterm delivery is likely, or delivery when maternal or fetal risk outweighs continued pregnancy.

The absence of a standardized decision threshold is crucial. An assay may flag a value below its reference interval, but that flag does not automatically mandate hospitalization or delivery. The obstetric team should explain whether the result adds anything beyond established findings and what specific follow-up it supports.

High hPL Results

High hPL most often reflects greater placental mass or altered maternal metabolism rather than a discrete disease. Multiple gestation is a common biologic explanation because two placentas or a larger shared placenta can produce more hormone. Larger placentas and higher infant birthweight have also shown positive associations in some populations.

Possible contributors include:

• twin or higher-order pregnancy;
• a large placenta or larger fetal size;
• maternal diabetes, insulin resistance, or obesity-related metabolic differences;
• inaccurate gestational age;
• normal variation near the upper end of late-pregnancy values;
• assay or unit mismatch.

A high value does not diagnose gestational diabetes. Diabetes screening uses a glucose challenge and, when indicated, an oral glucose tolerance test or other accepted diagnostic pathway. hPL helps explain why insulin resistance increases, but its concentration does not reliably separate normal adaptation from disease.

High hPL also does not confirm fetal macrosomia. Ultrasound estimates of fetal weight have their own margin of error, but they provide a direct size assessment. Maternal glucose control, prior birth history, fundal height, and serial growth are more useful than an isolated hormone result.

Very high values have been studied in relation to placental and fetal outcomes, but available evidence does not support a universal action threshold. In research cohorts, associations can differ after accounting for maternal body mass index, diabetes type, gestational age, and placental weight. A statistical association at the group level cannot predict one patient’s outcome with enough precision for stand-alone management.

If the result seems unexpectedly high, confirm fetus number and dating, review glucose testing, and verify units. An hPL value should not lead to dietary restriction or medication without evidence of a metabolic disorder. Pregnancy nutrition must support fetal growth; changes should be guided by glucose data and a prenatal clinician or dietitian.

hPL, Glucose, and Fetal Growth

hPL sits at the intersection of placental mass, maternal metabolism, and fetal nutrient supply, which makes it scientifically interesting but clinically difficult to interpret. A larger placenta may secrete more hPL, increase insulin resistance, and support a larger fetus. At the same time, maternal diabetes can enlarge the placenta and fetus through glucose-driven pathways that are not controlled by hPL alone.

In healthy pregnancy, rising insulin resistance is balanced by increased insulin secretion. Fasting glucose often remains normal or slightly lower because the fetus and placenta continually use glucose. After meals, glucose may stay elevated longer. Gestational diabetes appears when pancreatic compensation is insufficient.

Research has examined whether hPL predicts gestational diabetes, maternal insulin secretion, birthweight, or neonatal outcomes. Results have been inconsistent. A recent systematic review found relationships with placental mass and infant birthweight in some diabetes-affected pregnancies but insufficient evidence for hPL as a routine clinical biomarker. More recent observational work continues to explore metabolic associations, yet it has not replaced standard glucose testing.

For fetal growth, both low and high extremes can be relevant in theory:

• low hPL may accompany reduced placental mass and fetal growth restriction;
• high hPL may accompany larger placental mass and higher birthweight;
• maternal glucose, insulin, lipids, blood pressure, smoking, genetics, and placental blood flow modify both relationships.

The direction of causation is not always clear. hPL may be a signal of placental size rather than the driver of fetal size. In other situations, its metabolic effects may contribute. Human studies cannot easily separate these roles because hormone concentration and placental development change together.

For a patient, established measurements remain more useful: glucose values for diabetes, fundal height and ultrasound for growth, Doppler for placental resistance, and fetal movement or heart-rate surveillance for current well-being. hPL can add research context but rarely changes these decisions.

Better Ways to Assess the Placenta and Fetus

Placental function is not captured by one blood hormone. Modern obstetric assessment combines maternal health, fetal growth, blood flow, and signs of fetal adaptation.

Ultrasound growth assessment measures the fetal head, abdomen, and femur to estimate weight and growth percentile. Serial measurements are more informative than one scan because they show growth velocity.

Amniotic-fluid assessment provides indirect information about fetal urine production and chronic placental support. Low fluid can occur with placental insufficiency, membrane rupture, medication effects, or fetal urinary abnormalities.

Doppler ultrasound evaluates blood-flow patterns in vessels such as the umbilical artery, middle cerebral artery, and ductus venosus when indicated. Abnormal resistance can identify fetuses at higher risk from placental disease.

Nonstress testing and biophysical profile assess fetal heart-rate reactivity, movement, tone, breathing movements, and fluid. These tests estimate current well-being rather than long-term placental hormone output.

Maternal testing includes blood pressure, urine protein, blood counts, liver and kidney tests, and glucose screening based on the suspected condition. Biomarkers such as placental growth factor may be used in selected preeclampsia pathways, depending on country and guideline.

Contact the prenatal team promptly for reduced fetal movement, vaginal bleeding, fluid leakage, persistent headache, visual changes, upper abdominal pain, sudden swelling, contractions, or concerning blood-pressure readings. These signs require direct assessment, not an hPL test.

When reviewing an hPL result, ask three questions: Why was it ordered? Which validated interval applies? What action would change because of it? If the answer does not point to a clear evidence-based pathway, focus on the established maternal and fetal assessments that can guide care.

Historical interest in hPL came from its placental origin and its broad rise with placental mass. Researchers evaluated serial levels as a possible way to detect placental insufficiency or fetal growth problems. In practice, overlap between normal and complicated pregnancies, assay limitations, and the availability of ultrasound and fetal-surveillance methods reduced its clinical role. A single maternal concentration cannot show whether an individual fetus is receiving adequate oxygen or nutrients.

The metabolic effects of hPL are also easy to oversimplify. hPL contributes to maternal insulin resistance and fat mobilization, helping make nutrients available to the fetus, but gestational diabetes is not diagnosed from hPL. Glucose screening, oral glucose-tolerance testing, and established diagnostic criteria are used instead. A high hPL result does not prove diabetes, and a low result does not rule it out.

Placental size can influence hPL. Multiple gestation may produce higher concentrations, while a smaller or impaired placenta may be associated with lower values. The distributions overlap too broadly to diagnose twins, fetal growth restriction, or placental failure. If fetal growth is a concern, serial biometry, amniotic fluid, Doppler assessment, maternal blood pressure, and fetal testing provide more direct evidence.

Patients who receive an hPL value through a specialty or research laboratory should request the method, gestational reference interval, and intended use. Repeating the assay is worthwhile only if the specialist has defined how a change would alter care. Otherwise, established prenatal assessment should take priority.

References

• Human Placental Lactogen in Relation to Maternal Metabolic Health and Fetal Outcomes: A Systematic Review and Meta-Analysis 2022 (Systematic Review)
• Metabolic associations of human placental lactogen in pregnancies at high metabolic risk: An observational cohort study 2025
• Secretion of placental peptide hormones: functions and clinical implications 2025 (Review)
• Physiology, Placenta 2023 (Review)
• Gestational Diabetes 2024 (Review)

Disclaimer

This article provides general information about hPL and does not establish whether an individual placenta or fetus is healthy. hPL testing is uncommon and must be interpreted by the ordering clinician using gestational age, laboratory method, ultrasound, maternal health, and fetal surveillance. Contact obstetric care promptly for reduced fetal movement, bleeding, severe headache, visual symptoms, or significant abdominal pain.