Fertility Blood Test UK: What to Test Before Trying to Conceive, What Results Mean & Where the NHS Pathway Falls Short

1. When should you have fertility blood tests?

The short answer: earlier than the NHS pathway typically allows. The NICE CG156 fertility guideline recommends that couples who have not conceived after 12 months of regular unprotected intercourse should be offered investigation. That threshold drops to 6 months for women aged 36 and over, and earlier if there is a known clinical reason to suspect a problem.

In practice, this means the average couple waits 12–18 months before any blood work is done. For a 35-year-old woman, whose ovarian reserve is declining at its steepest rate, that is a significant window of time.

There are strong clinical arguments for baseline fertility blood tests before actively trying to conceive, particularly if:

• •You are over 30 and planning a pregnancy in the next 1–2 years
• •You have irregular or absent periods
• •You have a history of PCOS, endometriosis, or pelvic surgery
• •You have a family history of early menopause (before 45)
• •You have been on hormonal contraception for many years and are unsure of your baseline
• •You or your partner have had chemotherapy, radiotherapy, or testicular surgery

A pre-conception fertility blood test does not predict whether you will conceive. It identifies markers that fall outside expected ranges — giving you and your GP actionable information before time becomes a constraint.

2. Female fertility hormones: the complete panel

A comprehensive female fertility blood test covers three physiological domains: ovarian reserve (how many eggs remain), ovulation (whether eggs are being released), and hormonal environment (whether conditions support implantation and early pregnancy).

The NHS typically tests only FSH, LH, and progesterone in a standard fertility investigation. AMH, oestradiol, prolactin, and thyroid may not be included unless specifically requested or the woman is referred to a fertility clinic. This creates a gap: women with low ovarian reserve, subclinical thyroid dysfunction, or hyperprolactinaemia may wait months before these are identified.

3. AMH: ovarian reserve and what it actually tells you

Anti-Müllerian hormone (AMH) is produced by small antral follicles in the ovaries. It is the most widely used marker of ovarian reserve — a rough proxy for how many eggs remain. It does not measure egg quality, which is primarily determined by age.

AMH is uniquely useful because it can be tested on any day of the menstrual cycle. Unlike FSH, which fluctuates significantly across the cycle, AMH levels are relatively stable throughout the month, making it a practical screening test.

A critical nuance: a low AMH does not mean you cannot conceive naturally. It means your window may be shorter and that assisted reproduction (if needed) may require more aggressive stimulation protocols. Conversely, a high AMH (>35 pmol/L) may indicate polycystic ovary syndrome, which has its own implications for fertility.

The RCOG Scientific Impact Paper No. 63 confirms that AMH is the best single predictor of ovarian response to stimulation in IVF — but cautions against using it as the sole determinant of fertility potential. Women with low AMH and good egg quality (younger age) can still conceive; women with high AMH and poor quality (older age, other factors) may struggle.

4. FSH & LH: the pituitary signals that drive ovulation

Follicle-stimulating hormone (FSH) and luteinising hormone (LH) are produced by the pituitary gland and orchestrate the menstrual cycle. In the early follicular phase (days 2–5), FSH recruits a cohort of follicles; one becomes dominant. Mid-cycle, an LH surge triggers ovulation.

Day 2–5 FSH above 10 IU/L suggests the ovaries are working harder to recruit follicles — an early sign of diminishing reserve. Above 15 IU/L is more concerning. However, FSH can fluctuate significantly between cycles, which is why AMH is considered more reliable for reserve assessment.

The LH:FSH ratio also matters. In a normal cycle, FSH slightly exceeds LH in the early follicular phase (ratio ~1:1). An LH:FSH ratio >2:1 suggests PCOS, where excessive LH stimulates androgen production from ovarian theca cells, disrupting follicle maturation and ovulation.

Very low FSH and LH (<2 IU/L each) may indicate hypogonadotropic hypogonadism — where the pituitary is not sending adequate signals to the ovaries. This can result from extreme weight loss, over-exercising, chronic stress, or pituitary pathology. It is treatable but requires specialist investigation.

5. Oestradiol: follicle maturation and cycle quality

Oestradiol (E2) is the primary oestrogen, produced by developing ovarian follicles. In the early follicular phase (days 2–5), it should be relatively low — typically 100–300 pmol/L. An elevated day-3 oestradiol (>300 pmol/L) alongside a normal FSH can mask a declining reserve: the early oestrogen rise artificially suppresses FSH via negative feedback, making FSH appear “normal” when it would otherwise be elevated.

This is why fertility specialists interpret FSH and oestradiol together, not in isolation. A “normal” FSH of 7 IU/L with a day-3 oestradiol of 400 pmol/L is more concerning than an FSH of 9 IU/L with an oestradiol of 150 pmol/L.

Oestradiol also supports endometrial growth. Chronically low levels (as seen in hypothalamic amenorrhoea or premature ovarian insufficiency) result in a thin endometrium, reducing the chance of successful implantation even if ovulation occurs.

6. Progesterone: confirming ovulation actually happened

After ovulation, the collapsed follicle transforms into the corpus luteum, which produces progesterone. A mid-luteal progesterone (tested 7 days after presumed ovulation, or day 21 in a 28-day cycle) above 30 nmol/L confirms ovulation occurred. Below 16 nmol/L suggests anovulation or a short luteal phase.

The timing caveat is important. Day 21 is only correct if you ovulate on day 14 — which many women do not. In a 35-day cycle, ovulation might occur around day 21, making the correct progesterone test day 28. Testing too early yields a falsely low result. If you are unsure when you ovulate, ovulation predictor kits (LH urine strips) or basal body temperature tracking can help time the test.

The NHS typically offers a “day 21 progesterone” as the first-line fertility investigation. It is a useful test, but only confirms or denies ovulation in that one cycle. It tells you nothing about ovarian reserve, thyroid function, or hormonal environment. A single normal day-21 progesterone does not mean fertility is fine — it means you ovulated that month.

7. Thyroid & prolactin: the silent fertility disruptors

Thyroid dysfunction is one of the most treatable causes of subfertility, yet it is frequently missed in initial investigations. The NICE thyroid disease guideline (NG145) recommends TSH testing in women with fertility difficulties, but this is not always done in primary care.

Subclinical hypothyroidism (TSH 4–10 mIU/L with normal Free T4) has been associated with reduced conception rates, increased miscarriage risk, and adverse pregnancy outcomes. The American Thyroid Association 2017 guidelines recommend a pre-conception TSH target below 2.5 mIU/L for women planning pregnancy. Many UK labs use an upper reference limit of 4.0–4.5 mIU/L, meaning a TSH of 3.8 would be reported as “normal” despite being above the recommended pre-conception threshold.

Prolactin deserves specific attention. Mildly elevated prolactin (hyperprolactinaemia) can suppress GnRH pulsatility, leading to irregular or absent ovulation. Causes include stress, certain medications (antipsychotics, metoclopramide, SSRIs), and rarely pituitary adenomas. Prolactin should be drawn fasting, rested, and without nipple stimulation in the preceding 24 hours, as all of these can transiently elevate levels.

Read our full thyroid blood test guide for details on TSH, Free T4, Free T3, and thyroid antibodies.

8. Male fertility blood tests: testosterone, FSH & beyond

Male fertility investigation typically starts with semen analysis (not a blood test), which assesses sperm count, motility, morphology, and volume. If semen analysis is abnormal, blood tests help identify the cause.

The key male fertility blood markers are:

An important caveat: exogenous testosterone (testosterone replacement therapy, anabolic steroids) suppresses spermatogenesis via negative feedback on FSH and LH. If a man is using TRT and wants to conceive, he should discuss alternatives (such as clomiphene or hCG) with an endocrinologist. This is one of the most common iatrogenic causes of male infertility.

9. Reference ranges: what's normal vs what's optimal for conception

The gap between “normal” and “optimal” is clinically significant. A TSH of 3.5 mIU/L is within the NHS reference range but above the pre-conception threshold recommended by the American Thyroid Association and endorsed by the RCOG Green-top Guideline No. 80. Similarly, a day-21 progesterone of 20 nmol/L suggests ovulation occurred but may indicate a suboptimal luteal phase.

10. Cycle timing: when to take each test for accurate results

Fertility hormone levels change dramatically throughout the menstrual cycle. Testing at the wrong time can produce misleading results. The key windows are:

If your periods are irregular or absent, testing can be done on any day — but note this when interpreting results. Your GP or fertility specialist may want to repeat tests in a subsequent cycle for confirmation. AMH is the least affected by cycle timing and is the most practical single test for women with irregular cycles.

11. The NHS fertility pathway: what gets tested and what gets missed

The NHS fertility investigation pathway is outlined in NICE CG156. It is a clinically sound framework, but it is designed for resource-constrained public healthcare, not proactive fertility planning.

The total time from “we want a baby” to a complete fertility workup can easily be 18–24 months through the NHS. For couples where female age is a factor, this delay has real consequences. A BMJ study (2012) found that fecundity (monthly probability of conception) drops from approximately 25% at age 25 to 10% at age 35 and below 5% at age 40.

Private fertility blood testing does not replace the NHS pathway — tubal assessment, ultrasound, and semen analysis still require clinical facilities. What it does is give you the hormone picture 12–18 months earlier, so that conversations with your GP start from a position of knowledge rather than uncertainty.

12. GP blood test vs Helvy: what's the difference?

The NHS pathway is comprehensive once you enter it. The value of private testing is in the timing — getting baseline information before the 12-month clock starts, identifying treatable issues (thyroid, prolactin) early, and having data to bring to your first GP conversation about fertility.

13. Common result patterns and what they suggest

14. Frequently asked questions