Fertility Calculator UK
Calculate your ovulation date, fertile window, next period, and a 3-month fertility calendar based on your menstrual cycle.
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Understanding Your Menstrual Cycle
The menstrual cycle is divided into four phases, each driven by changing hormonal levels. Understanding these phases is fundamental to tracking fertility and recognising signs of ovulation.
Phase 1: Menstruation (Days 1–5, average)
The uterine lining (endometrium) sheds, triggering your period. Oestrogen and progesterone are at their lowest. This marks Day 1 of your cycle.
Phase 2: Follicular Phase (Days 1–13, overlaps with period)
The pituitary gland releases FSH (follicle-stimulating hormone), stimulating several ovarian follicles to grow. One becomes dominant, producing increasing amounts of oestrogen. The rising oestrogen thickens the uterine lining and triggers an LH surge. This is the most variable phase — its length determines cycle length variation.
Phase 3: Ovulation (Day 14 in a 28-day cycle)
A surge in LH (luteinising hormone) causes the dominant follicle to rupture, releasing a mature egg. The egg survives for 12–24 hours and must be fertilised within this window. This is the only time conception is possible directly — though sperm deposited in the days before can fertilise the egg as it is released.
Phase 4: Luteal Phase (Days 15–28, average)
The empty follicle becomes the corpus luteum, producing progesterone. Progesterone maintains the uterine lining for potential implantation. If fertilisation does not occur, the corpus luteum degenerates after 12–14 days, progesterone drops, and menstruation begins. The luteal phase is typically consistent at 12–14 days.
Cervical Mucus Changes Through Your Cycle
Cervical mucus changes in texture and quantity throughout the cycle, providing reliable fertility signs:
- After period (Days 5–7): Dry or minimal mucus — low fertility.
- Pre-fertile (Days 8–10): Sticky, cloudy mucus — fertility increasing.
- Fertile (Days 11–14): Creamy, white, lotion-like mucus — approaching ovulation.
- Peak fertility (Days 12–15): Clear, stretchy, egg-white consistency (EWCM — egg white cervical mucus) — ovulation is imminent or occurring. This is the most fertile sign.
- Post-ovulation: Returns to sticky or dry — low fertility.
Age and Fertility Decline in Women
| Age Range | Monthly Conception Chance | IVF Success Rate (NHS, per cycle) | Miscarriage Risk |
|---|---|---|---|
| 20–24 | 25–30% | ~35% | ~10% |
| 25–29 | 20–25% | ~32% | ~12% |
| 30–34 | 15–20% | ~28% | ~15% |
| 35–37 | 10–15% | ~25% | ~20% |
| 38–40 | 8–10% | ~19% | ~30% |
| 41–43 | 5–8% | ~11% | ~40% |
| 44+ | 1–3% | ~4% | ~50%+ |
IVF success rates from HFEA (Human Fertilisation and Embryology Authority) data, 2023. Monthly conception chances are population averages for couples with no known fertility issues.
NHS Fertility Guidance: When to Seek Help
The NHS recommends consulting your GP if you have not conceived after:
- 12 months of regular unprotected sex if you are under 35
- 6 months if you are 35 or older
Seek earlier advice if you have: irregular or absent periods, known PCOS or endometriosis, a history of pelvic inflammatory disease, previous surgery involving the fallopian tubes or uterus, or if your partner has previously had fertility problems. Your GP can arrange initial tests including blood tests (FSH, LH, AMH, oestradiol on Day 2–5 of cycle; progesterone on Day 21), pelvic ultrasound, and semen analysis.
IVF in the UK: HFEA Statistics
The Human Fertilisation and Embryology Authority (HFEA) regulates fertility treatment in the UK. In 2023, approximately 52,000 IVF cycles were performed in UK fertility clinics. Key statistics:
- Overall IVF success rate (all ages): approximately 23% per embryo transfer
- Success rate using donor eggs (any age): approximately 28% per cycle
- NHS funding: eligibility varies by Clinical Commissioning Group; NICE recommends up to 3 cycles for women under 40 with unexplained infertility of 2+ years
- Frozen embryo transfer cycles have comparable or slightly better success rates than fresh cycles
- Add-on treatments (endometrial scratch, time-lapse imaging): limited evidence; HFEA rates most as 'amber' (not enough evidence to recommend)
Sperm Health and Male Fertility
Male factor infertility accounts for approximately 30–40% of fertility problems in the UK. A normal semen analysis (WHO 2021 criteria) requires: volume ≥1.4ml, concentration ≥16 million sperm/ml, total motility ≥42%, progressive motility ≥30%, normal morphology ≥4%. Key factors affecting sperm health include: smoking (reduces count and motility by up to 24%), alcohol (>14 units/week impairs testosterone production), heat exposure (laptops, hot baths, tight clothing — sperm production is optimal at 34–35°C, below core body temperature), anabolic steroids and testosterone supplements (cause severe suppression of sperm production), stress, and obesity (excess fat converts testosterone to oestrogen).